PROTOSCHIZODUS AXINIFORMIS. 231 



genus. There are two specimens of this variety in the Collection of the Geolo- 

 gical Survey, Jermyn Street, which I have been kindly permitted to figure 

 (PI. XVII, figs. 11 and 12), and which I describe below. 



The name Amphidesma axiniformis was given to the shell which I regard as 

 typical of the species, and which I retain, although the Schizodus axiniformis, 

 Phillips, sp., has been confused with Portlock's shell by various authors. There 

 is, however, no evidence at all that Portlock had Phillips's shell in his mind, but 

 the very reverse; for a few lines lower down {op. cit., p. 439), under " Amphi- 

 desma deltoidea," he says, " Phillips suggests of his species that it may be the 

 young of Isocardia axiniformis, but such reference appears very doubtful." The 

 figured specimens are preserved in the Geological Survey Museum ; but the 

 specimen labelled Amphidesma depressa is not the figured specimen, which is, 

 however, present in the Collection. 



The type of ^4. axiniformis, Portlock, is represented in the original drawing as 

 having its anterior umbonal slope angulated. This seems to me to be an error, 

 and my artist's drawing (PI. XVII, fig. 10) shows the absence of this character. 



Portlock states of this series of shells, as quoted above, the three forms " may 

 prove varieties of the same species " — A. deltoidea, Portlock ; the internal cast, 

 was, however, considered as a distinct species. 



Unfortunately I have not been able to obtain any more examples from the 

 locality whence the types were obtained, and am unable to decide, owing to the 

 absence of sufficient material, whether the A. depressa, Portlock, is really more 

 than a varietal form ; I have therefore described it as a variety of P. axiniformis. 

 I have not, however, met with similar varieties in other localities where this 

 species occurs. 



Although, unfortunately, I have not been able to isolate the hinge in any 

 specimen, I have assigned this shell to the genus Protoschizodus, de Koninck, upon 

 the evidence of the muscle-scars and external characters, which agree in every 

 detail with those shells known from their hinge characters to belong to the 

 genus. 



The description {op. cit., p. 129) and figures of P. Wortheni given by de Koninck 

 leave no room to doubt that his shell is identical with Portlock's type, although 

 he states, " elle a quelque ressemblance avec le P. {Amphidesma) axiniformis, J. E. 

 Portlock, dont il a a peu pres la taille, mais le cote posterieur est plus fortement 

 tronque et son extremite est plus anguleuse." A comparison of the figures of 

 de Koninck's specimens with those which I give will show, however, that this 

 difference does not exist. 



The hinge characters were, it appears, described from this species. 



Brown was evidently of opinion that there was some coufusiou in the nomen- 

 clature, for when he published his ' Illustrations of Fossil Conchology ' he gave 



