INVERTEBKATE PALAEONTOLOGY. 297 



margin, marked by very tine, closely-set, obscure, concentric striae; internal 

 casl showing Taint traces of irregular radiating stria', and obscure indications 

 of the horse-shoe-shaped muscular sear. 



Diameter of base, 1.22 inches; height of apex, about 0.74 inch. 



This shell has very much the form and general appearance of a species 

 described by Professor Forbes from the Cretaceous rocks of Southern India, 

 under the name Ca/yptra-a elevata* I infer, however, from Dr. Stoliczka's 

 remarks in regard to the Indian shell, after examining the type-specimen in 

 'the collection of the Geological Society at London, that it is thicker than our 

 species, as he says that " the shell itself is rather thick as compared with 

 Calyptreea and Anisomyon. v f His figure also shows it to have a slightly more 

 depressed form than our species, while it is without any indications of the 

 small nipple seen at the apex of the internal cast of the latter. The two 

 shells, however, must be very nearly related ; and I should have made more 

 critical comparisons in first studying our type, had it not been for the fact 

 that Professor Forbes had referred his species to the genus Calyptreea, while 

 ours is clearly, and beyond all doubt, not a Calyptraza. Dr. Stoliczka, how- 

 ever, says that Professor Forbes's species is also certainly not a member ot 

 that genus, as he could discover, in the cast of the interior, no traces of the 

 internal process of Calyptnea.% Although not impossible, therefore, that 

 these two forms may yet prove to be specifically identical, this is still improb- 

 able, owing to their widely distant geographical positions. 



Although it is certainly not a Calyptreea or Capulus, I am still in doubt 

 in regard to the generic relations of this species. Its summit is much more 

 elevated than is usual in Acmcea, the form and general appearance being much 

 the same as in Scurria, Gray ; and I am only prevented from calling it 

 Scurria fragilis by its greater thinness than we usually see in that genus. 

 In removing it to the genus Acmcea (— Tectura), it became necessary to give 

 it a different specific name, since the name A. fragilis had been previously 

 applied to a recent species of the genus. 



Locality and position. — Fox Hills, Dakota ; from the Fox Hills group of 

 the Upper Missouri Cretaceous series. 



* Trans. Oeol. Soc. Lond., VII (2d ser.), pi. 12, fig. 1U. 



t PalsBout.Indica,II,322. 



} Dr. Stoliczka refers tin- Indian species doubtfully to Tectum (= Acmaea). 



38 n 



