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Notocallista (Striacallista) pestis n.sp. Plate 13, figs. 3, 4.
Shell about average size for the subgenus, suboval to subtrigonal, narrowed posteriorly, beaks moderately conspicuous, rather narrow. Surface shining, posterior and anterior parts bearing concentric grooves and ridges, about 4 per mm., but these are very irregular and die out over the middle of the disk. Ligament shallow, its walls low. Hinge close to that of multistriata. Pedal retractor close to the adductor but separated from it. Pallial sinus roundly truncate.
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Holotype in Auckland Museum (ex Finlay Collection).
Length, 27 mm.; height, 19 mm.; inflation (1 valve), 5.5 mm.
Locality: 400–500 ft., Abattoirs bore, Adelaide. Age uncertain.
Although the holotypes of N. pestis and N. molesta, which come from the same locality, are, on the criteria used in this paper, almost, if not quite, subgenerically separable, paratypes seem to show a gradation between the two species. Of the 9 specimens examined from the Abattoirs bore no clear-cut groups could be formed. The only two characteristic specimens of N. mollesta have a sunken ligament and persistent sculpture. They also have a fairly regularly oval outline. The other specimens have persistent sculpture and an oval shape, but they have a shallow ligament and so have been classed under pestis. The other five specimens with shallow ligaments and obsolescing sculpture differ considerably among themselves in shape, in the pallial sinus, in the degree of separation of the pedal retractor from the adductor, and of the left posterior cardinal from the nymph. The deep ligament and separated left posterior cardinal of the type of N. mollesta are characters of Fossacallista, but the pedal retractor is not confluent and the pallial sinus is roundly truncate. Thus as well as showing mixing of specific characters, the Abattoirs bore collection shows blending of the sub-genera Fossacallista and Notocallista. The most convenient refuge at present is to explain the mixture as due to hybridisation of stocks, but it is somewhat disturbing to find characters that apparently have generic (or subgeneric) significance in the rest of the family, yet that cannot show consistent specific divisions in this small group.
The specimens were obtained from some 100 ft. or so of beds in a bore, and probably more than one horizon is represented (N. H. Woods, 1931, p. 147), but this does not explain away all the anomalies.
N. pestis agrees closely with multistriata in shape, sculpture, and hinge, the most consistent difference being the more sharply truncate sinus of multistriata.