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2. Genus Struthiolarella Steinmann and Wilckens, 1908.
Genotype: Struthiolaria ameghinoi von Ihering.
This group was separated from Struthiolaria as a subgenus (Steinmann and Wilckens, 1908, p. 53) for the reception of the South American species, on the grounds that they differed from the typical New Zealand shells as follows: (1) “On the older whorls spiral sculpture does not predominate, but axial ribs, which are, it is true, crossed by fine spirals”; (2) “there is no continuous spiral angle formed on the upper part of the whorls.” Other important features justify the separation. Ortmann's figure of S. ornata (1901, pl. 33, fig. 12a), reproduced below (text-fig. 6, a), shows convex-whorls with the curved axial ribs crossed by spirals as in Monalaria,
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while on the base are two strong spiral cords. Other and more developed species of the group show the axials abbreviated to rounded tubercles with many strong spirals below. The columella is in most cases only slightly bent; but in S. nordenskjoldi Wilckens the curvature is marked, and the callus is well developed, showing that a gerontic stage has been reached. In all cases the outer lip has the same contour as that of Monalaria—i.e., there is one prominent sinuosity.
The development of strong spirals on the base shows that this group branched off from Monalaria sensu lato before the development of such as M. concinna, but it may have come through M. minor. The age of these two species cannot definitely be placed on the European time-scale, but the probabilities are that the latter is about Palaeocene and the former Oligocene. The curvature of the columella and the spread of the callus in mature
Fig. 6.—a, b. Struthiolarella ornata (Sowerby). (After Ortmann.)
c. S. ameghinoi von Ihering. (After Ortmann.)
d, e. S. nordenskjoldi Wilckens. (After Wilckens.)
individuals—see Steinmann and Wilckens, 1908, figures of S. ameghinoi, pl. 6, fig. 7, and S. nordenskjoldi Wilckens, 1911, pl. 1, figs. 26, a, b—in addition to the strong basal spirals, justify the generic separation of Struthiolarella from Monalaria, while the contour of the outer lip and the ornamentation separate it from Struthiolaria. Wilckens later (1922, p. 17) tentatively suggested that S. nordenskjoldi was wrongly classed with S. ameghinoi, and was more closely related to Conchothyra parasitica. The writer does not agree with this, and considers S. nordenskjoldi to be a gerontic development of Struthiolarella.
Under Struthiolarella, Steinmann and Wilckens included Tylospira coronata (Tate) from the Lower Tertiary of Victoria, and the living Struthiolaria mirabilis Smith from Kerguelen Land, not granting generic recognition to Tylospira as based on B. scutulatum. Previously Tate (1889, p. 170) had included the Kerguelen shell with T. coronata and T. scutulata in his interpretation of the genus Pelicaria (i.e., Tylospira).
Fig. 7. Struthiolarella mirabilis (Smith). (After Tryon.)
Though the apertural callus is lacking, S. mirabilis certainly is similar to S. ameghinoi, and is perhaps rightly associated with Struthiolarella; but
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the case for the inclusion of T. coronata is not so good, because it involves the separation of that species from T. scutulata and T. clathrata.
It is necessary, before going further, to determine what relative importance should be conceded to the various shell-characters. Roughly, the order of importance may be stated as—first, the formation of the aperture; second, the ornamentation; third, the disposition of the callus. (The protoconch is, of course, very important in classification, but the material available does not allow of its use in the present case.) Naturally, the rule cannot be applied absolutely, because a small difference in the aperture might not carry the same weight as a considerable difference in sculpture; also, the possibility of parallelism and convergence must be taken into account. Still, there is a broad relative value attached to the features mentioned.
Fig. 8.—a. Tylospira coronata (Tate). (After Tate.)
b. Tylospira scutulata (Martyn).
As regards the generic position of T. coronata, an examination of actual specimens shows that it cannot be separated generically from T. scutulata, the type of Tylospira. Both species have a bisinuous outer lip which is not reflexed in the adult. In late youth this lip acquires a shining callus which, continuing across the suture, ascends the wall of the preceding whorl. Unlike Struthiolaria, growth continues for a considerable time after the formation of this callus, so that no ornamentation except growth-lines and a few obsolete spirals is developed on the body-whorl. If Steinmann and Wilckens were correct in classing T. coronata as Struthiolarella, then T. scutulata would also have to be included, and Tylospira would supersede Struthiolarella. The former genus, however, has a bisinuous outer lip, while the latter has a unisinuous one, so that the two generic terms should stand, Tylospira for the Australian and Struthiolarella for the South American species.