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The generic term Pseudomonotis has been used in a very loose sense to include upper Palaeozoic and Mesozoic shells of many shapes and sizes and kinds of sculpture, but all characterised by an anterior byssal ear and notch in the flattened right valve. Some attempts have been made at subdivision, but the major groups, except Oxytoma Meek, have not gained generic recognition. Of course, no rules can be laid down as to what constitutes a genus, and the matter comes in the end to be one of individual taste. Nevertheless, utility must have a strong influence, and it is because of a belief in their strati-graphic and palaeogeographic as well as of their systematic significance, that the new genera here described have been proposed.
Owing to the lack of most of the European palaeontological literature, a thorough survey of the different groups cannot be carried out in New Zealand, but since two of them are represented here, one being widespread, their nomenclature has to be dealt with.
Pseudomonotis was introduced by Beyrich in 1862, but his paper is not available in New Zealand, so the original species cannot be cited here. The genus soon became widely comprehensive, no
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uncommon happening, but an unusual feature is that in strati-graphical and palaeontological text-books any one of three distinct groups may be found illustrating the “genus.” These groups are:—
(1) The Permian species related to P. speluncaria Schlotheim (Figs. 5, 6). They are of moderate size, of subtriangular shape, and are not strongly inequilateral, having a relatively short hinge-line and a weak, bluntly rounded, posterior ear. The sculpture consists of more or less irregular, imbricate ridges, tending to differ much in strength on the two valves.
(2) The Upper Triassic species such as P. ochotica (Keyserling), subcircularis (Gabb), and richmondiana (Zittel) (Figs. 9, 11). These are large, highly inequilateral shells with a relatively long, straight hinge-line and a well-developed, obtusely angled, posterior wing which has a gently curved posterior margin. The sculpture is regular, consisting of strong radial ribs on both valves, often with interstitial riblets.
(3) The Jurassic species belonging to the group of P. echinata W. Smith (Figs. 7, 8). These are rather small shells, having a sub-quadrangular outline and a very long, straight hinge-line. The posterior ear is well developed, having a deeply concave posterior margin which makes an acute angle with the dorsal margin.
In addition to these, a number of species, now attributed to Pseudomonotis, have been described from the Lower Triassic of Europe. They are rather small, plump shells having a fairly long hinge-line and a very small byssal ear. The sculpture consists of fine radials, not imbricate, and tending to become obsolete. These have already been subdivided into two groups, Claraia Bittner, and Eumorphotis Bittner, presumably as sections of Pseudomonotis, which is the rank given by Diener.
The chief link used to bind these and other groups in the one genus is the byssal ear and notch in the right valve, but other groups such as the Pectens and Buchias, possess similar structures. Moreover, the different groups of “Pseudomonotis” present important differences in the arrangement, not only of the anterior ear and notch, but also of the ligament. It is therefore clear that, as generally conceived, Pseudomonotis includes widely separated lineages, and generic ranks for the chief groups indicated is more in accord with systematic and stratigraphical demands.