20 AMEEICAN JOUENAL 



would be apt to confound Pecten and Avicula, but it is not so 

 easy to decide where to place the carboniferous Aviculopecten or 

 Eumicrotis. 



In the present instance the greatest difficulty lies among 

 those shells that have been usually described as Pteroccra. Since 

 the establishment of Alaria by Morris and Lycett, many of them 

 have been placed under that genus, hardly more appropriately 

 than under Pterocera, and without contributing at all towards 

 the solution of the difficulty. As originally described, and as 

 accepted by Pictet, Piette and others, Alaria includes all the 

 doubtful forms more or less fusiform and resembling (and many 

 very distinct from) Postellaria, but without a posterior canal. 

 Some of those possess well marked varices, the results of arrests 

 in growth, like those of Ranella, while others are entirely 

 destitute of them. For the first of these two classes I have pro- 

 posed to retain Morris and Lycett's name ; for the other I sug- 

 gested the name of Dicroloma. This is a division warranted by 

 the views of all Conchologists ; the presence or absence of varices 

 being, in the great majority of cases, a character of even greater 

 than generic value. The authors of the genus proposed to include 

 everything that could be placed in, what they considered, tStrom- 

 bidoi, and that simply differed from their views of Rostelkma 

 and Ptevoeera^ in the absence of the posterior canal running 

 up the spire. In restricting the genus, therefore, it is necessary 

 to follow the usual rule of ascertaining what are the characters 

 of the first species described under the generic name. That 

 species is A. armata, which forms a tri-digitate lip when young, 

 that lip remaining as a varix on the older shell. My name 

 Dicroloma, therefore, must be applied to the allied shells, fusi- 

 form in shape, without posterior canal, and with a bi-, tri- or multi- 

 digitate outer lip and thin inner lip. 



Since the publication of the paper above referred to, I have 

 received from my friend. Dr. Ferd. Stoliczka, the first part of 

 his admirable work on the Gasteropods of the cretaceous forma- 

 tion of India. He there goes over the question very ably, and 

 proposes what he admits is only an artificial and temporary 

 classification. I cannot concur with the Doctor in all the views 

 he has advanced, and propose, after copying his table, to point 

 out wherein we differ, and my reasons for holding other views. 

 He proposes : 



" 1st. To restrict the name Alaria to the species with a simple 

 undivided and narrow wing, as the Jurassic Al. Jiamus, Desh., 

 and Al. rhinoceros, Piette and Desh. 



^'2d. Species which have the exterior termination of the wing 

 extended in two opposite directions, (as llostellaria carinata, 



