[Read before the Wellington Philosophical Society, 16th January, 1874.]
At the suggestion of our late President, Dr. Hector, I have examined microscopically the fossil teeth of the remains of the Leiodon in the Museum.
The aquatic Saurians are arranged under Sauropterygia and Pythonomorpha. The former had two pairs of limbs, the latter an anterior pair only. The Leiodon belongs to the Pythonomorpha, with snake-like bodies of immense length. The L. dyspelor, discovered in New Mexico, is estimated at not less than 100 feet in length, and would be, says Professor Cope, the longest reptile known, and may well excite our astonishment.
The Leiodon is closely allied to the celebrated gigantic Mosasaurus hoffmanni, or what was at first called the crocodile of Maestricht. Neither Mantell nor Owen were able to say, from the few and scattered remains to which they had access, whether the Leiodon is a species of Mosasaur or a distinct genus. The chief distinction is in the teeth, which, in the Mosasaur, have the outer side flat with two sharp edges, while the inner side is round. Where the teeth are absent the unsettled distinction between the Mosasaur and the Leiodon renders it probable that some of the former species may really be Leiodon, as suspected by Professor Cope in his paper on the fossil reptiles of the cretaceous
rocks of Kansas. You will not be surprised to find that the fossil remains in the Museum were first ticketed as “Mosasaur.”
Those who have read Mantell's “Wonders of Geology” will recall to mind that Mons. Hoffman discovered the remains of the Mosasaur in the quarries of St. Peter's Mount, in the suburbs of Maestricht; how he was despoiled of his specimen by the greedy canon of the cathedral; how, when the armies of the French Republic advanced to the gates of Maestricht and the town was bombarded, the troops were not allowed to play on that part of the city in which the celebrated fossil was known to be contained; and how, when the city was taken, the canon had to give up his ill-gotten prize, which was immediately transmitted to the Jardin des Plantes at Paris, “where,” says Mantell, “it still forms one of the most striking objects in that magnificent collection.” We may regard with pride and exultation the remains of the Leiodon in our Museum—a trophy of geological research, not a spoil of war!
The enormous jaw-bone of the Mosasaurus hoffmanni measured 4 ½ feet long; ours, of the Leiodon, would be less than that. I shall, in this brief paper, confine my remarks principally to the examination of the teeth, this being the work I have undertaken as a supplement to the paper by Dr. Hector.*
It is observed by Owen that the value of dental characters is enhanced by the facility with which they may be rendered available to the palæontologist in the determination of the nature and affinities of extinct species, of whose organization the teeth are not unfrequently the sole remains. I am not, therefore, disposed to undervalue the importance of the subsidiary work I have been engaged in. Mantell, by comparing the fossil teeth with those of recent Lacertæ, was able satisfactorily to place gigantic fossil remains of Tilgate Forest among the extinct species of the Pleurodont section of Iguanians. Again, among the Saurian reptiles, Owen remarks that hitherto in investigating the internal structure of the teeth of the crocodile, Plesiosaur, Myalosaur, Monitor, and more recent Lacertians, he had found the dentine body of the tooth to consist of calcygerous tubes radiating direct from the pulp cavity at right angles to the external surface of the tooth; but, in the Labyrinthodon, he found the most singularly complicated convolutions of the dentine. Through the kindness of the Hon. Walter Mantell, I am able this evening to offer you for microscopic inspection a valuable section of the tooth of the Lab. jaegeri. I could not show you a more interesting proof of the value of dental characters. I have here, also, a section of the human tooth, in which you will be able to trace the fine calcygerous tubes which form the minute structure of the teeth of all Vertebrates, and which, by their uninterrupted
[Footnote] * See Art. LII.
passage from the centre to the periphery of the teeth, unquestionably establish the view that the substance of the tooth is not built up by successive deposition of layer after layer of bony corpuscles, as maintained by Cuvier and the “excretion” theorists, but that the growth of the tooth is carried on by spreading tubuli, through which the nutrition is preserved and necrosis and absorption effected.
It is not the teeth only which afford interesting links in the chain of vertebrate animals. The structure of the jaw-bone is remarkable in the Lacertæ. It is built up of three or four bones, adjusted and anchylosed together; and it has been somewhat fancifully supposed by Buckland and Owen that this arrangement gives, in the case of the Crocodile and the Ichthyosaurus, additional strength to the jaw, and renders it better able to resist the violent concussion of their formidable mandibles when snapping at their prey.
In the mandibular bone of the Leiodon, it will be seen from Pl. XXV. that it is made up of four bones spliced together, viz., the dentary, the coronoid, the angular, and the splenial or opercular. The section from which the drawing is taken at once reveals to us that it formed part of the fossil remains of a Lacertian.
In the Lacertæ the jaw-bone, in most cases, presents only a sort of parapet on the outer side, and the teeth are fixed to it by a bony mass occupying the place of their root, and incorporated organically both with the tooth and the jaw-bone. In the Mosasaurus and the Ichthyosauri there is an inner parapet as well. In the Leiodon there is both an inner and outer parapet, with a deep fossa between, as seen in Pl. XXV.
The crown of the tooth in the Leiodon has a simple conical form. It is polished, striated, and of a dark colour. The numerous fine longitudinal striæ sharply marked on the polished surface of the tooth are owing to the splitting down of the crusta petrosa. The slits are well shown in Pl. XXIV. They do not extend to the dentine beneath. The margins, separating the outer from the inner, are well defined, and are sometimes broken by slight elevations, but this irregularity in no case gives a dentated character to the outline. Pl. XXIV. B shows the outline of a transverse section of the middle of the crown of a tooth.
The base, or fang, of the tooth presents a tapering subventricose form, and is implanted, as in the Mosasaur, in a cementing alveolus, raised in a rounded form from the deep longitudinal fossa formed by the ridges or parapets of the dentary bone (Pl. XXV.). The teeth, although hollow, do not, like those of the crocodile, contain in themselves the replacing teeth. Owen describes the teeth of the Leiodon as supported on a hillock of bone resting upon the broad alveolar surface of the jaw; but, in fact, the hillocks, or cementing
alveoli, rest on the thin floor of the dentáry fossa, to whose parapets they A are anchylosed, as shown in Pl. XXV.
The Leiodon belongs neither to the Pleurodonts, in which the teeth are attached to the inner side of the dentary bone, or, according to Owen, to an exterior alveolar plate of bone, the inner plate not being developed; nor to Pleodonts or to Cælodonts of Dumeril and Bribon, the former of which have solid teeth, attached by their bases to the groove on the inner side of the dentary bone, and the latter have hollow teeth, applied like buttresses against the outer plate of the dentary bone. The distinctive character of the-teeth of the Leiodon is their position on elevated cementing alveoli or hillocks; hence the term Acrodonts, applied to this Lacertian and the Mosasaurs.
The fang of the fossil tooth is sharply defined in the vertical section of the jaw-bone (Pl. XXV.). The sides become thin and evanes Acent below, and the extremity, instead of being pointed, forms the wide opening of the pulp-cavity.
The expanded base of the germ of the successional tooth is developed from the inner side of the dental fossa, in the interspaces of the primitive teeth, and from within the cementing alveoli. In its growth the young tooth shoots through the alveolus, and, pressing against the alveolus adjoining the base of the primitive tooth, interferes with its contour, and at length loosens it, and causes, both it and its tooth to fall. It will be seen, in Pl. XXIV. A, that the germ springs from the floor of the dental fossa, and stands with its broad base in a cavity communicating with the pulp-cavity of the primitive tooth; so that it would seem as if the young tooth was growing in a vascular, or, at any rate, not a fully ossified pulp, which, in the fossil state, has become replaced by a homogeneous deposit extending up the pulp-cavity of the adjoining primitive tooth. The germ touches the dentary bone, and must have sprung from a vascular membrane covering the floor of the dental fossa, and lining the pulp-cavity of the full-grown protruded tooth.
In order to obtain satisfactory views of the structure of the teeth, vertical and transverse sections were made. The transverse section, Pl. XXIV.F, shows, besides the dentine, an outer layer, the crusta petrosa, of extreme tenuity, being less than .03 inch thick. The divisional lines, forming block-like masses, are seen on the surface of the tooth as longitudinal striæ, as mentioned above. The dentine consists of a single pulp-cavity and a system of excessively fine calcygerous tubuli at regular and minute intervals from each other, radiating from the pulp-cavity at right angles to the periphery of the tooth. The walls of the tubuli are diaphanous, and the interspaces occupied by a bony or calcified deposit, giving a somewhat cellular appearance to the dentine (E), with occasionally a pinnate arrangement of short branches shooting from the tubuli nearly across the intervening spaces. Owen notices a similar appearance in the tooth of the Mastodon.
The pulp-cavity is filled, in the fossil state, by a deposit of the matrix. In one of the specimens a crack passes through the tooth into the cavitas pulpi, and has become filled, along with the latter, with a homogeneous deposit.
The thin crusta petrosa also consists of nearly diaphanous corpuscles, with numerous traces of partly effaced tubuli, so as to give ample assurance that this investing coat, like the rest of the tooth, is built up of calcygerous tubuli and calcified deposits; but, that the tubuli are no longer continuous with those of the dentine, or have any relation to the nutrition of the tooth—merely, in fact, serving as an inert, hardened crust, to protect the living dentine beneath.
There is no clear line of demarcation between what has been called the enamel and the dentine. It cannot be seen where the one begins and the other terminates. A microscopic examination shows that the tubuli (which do not exceed in diameter .0002 inch), together with the calcareous deposit in the intertubular spaces (averaging .0012 inch), form the substance of every part of the tooth—crusta petrosa, enamel, and dentine—each of these being merely modifications of continuous calcygerous tubes and bony corpuscles, of which only the crust has lost the properties of living matter. Owen must entertain the same view, although he is evidently guarded in his expressions. In his remarks on the microscopic examination of the teeth of the elephant, he notices that the tubuli of the crusta petrosa (or cement) appear to be directly continued from the tubuli of the ivory, although Retzius had expressly denied the continuation; and again, in his remarks on the fossil teeth of the Mastodon, he states that the minute terminations of the calcygerous tubes of the ivory are directly continued into the system of fine parallel tubes of the cement (crusta petrosa).
Towards the centre of the teeth, in a transverse section, concentric contour lines are seen, caused by an opaqueness of the intertubular deposit (F). Separation of the fossil tooth into superimposed layers takes place along these more opaque portions of the dentine, directly across the course of the calcygerous tubes, as in the fossil teeth of the elephant. Owen alludes to this in his description of the tooth of the Leiodon, and states that the concentric arrangement of the lamellæ, arising from the decomposition of the tooth, has been used as an argument in support of the untenable “excretion” theory.
The iridescent lustre of the polished surface of the transverse section is owing to the play of light on the reflecting walls of the radiating tubuli.
A vertical section exhibits a pseudo-cellular arrangement. The edges of the calcygerous tubes refract the light in short, brilliant lines, or appear as dark bars, about .001 inch in length. Where the section passes through the tubuli in a more or less oblique direction, a singular appearance, which I have attempted to represent in Pl. XXIV. F, shows itself.