338 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



Another exceptional shell is Callista vulnerata, Brod., a rather rare 

 form from the Pacific coast of America. This has the external aspect 

 of a Callista, and was so classed by Romer, but inside it presents the 

 dentition of Pitaria, and was properly referred to that genus by 

 Dr. Dall in 1902.^ Moreover, as pointed out by the latter, it is 

 remarkable for having its inner margins irregularly crenulated, 

 a feature not found in any other member of either genus. On 

 account of tliese peculiarities I propose to separate it as a sub-genus 

 of Pitaria under the name of Callizona, from its purple marginal band 

 of colour, which is often reduplicated as a zone along the lines of 

 growth. 



A more important and inconvenient exception to the discrimination 

 of Amiantis aud Pitaria, as above formulated, is that of Pitaria 

 tumem, the very type of the latter genus ; for in this shell the left 

 posterior cardinal resembles that of Amiantis in being confluent with 

 the nymph along its whole course, and in this respect differs from 

 most other species of Pitaria. There is also another point of difference 

 between P. tumens and its congeners, this being the form of the 

 pallial sinus, which is long, horizontal, linguiform. and pointed at 

 the anterior end, like that of Amiantis callosa and A. dione. 



Pitaria tumens therefore combines some of the features of Amiantis 

 with some of those which are distinctly characteristic of most other 

 Pitaria, but it must remain the type of the latter because it is the 

 original ' Pitar^ of Adanson. We are therefore in this predicament, 

 that the majority of the species composing the genus Pitaria differ 

 from the genotype in two important particulars, i.e. in the position 

 of the left posterior cardinal and in the possession of a sliort rounded 

 or bluntly angular sinus ; while the genotype only differs from 

 a typical Amiantis in certain external characters, such as its finer 

 surface sculpture, the exsert lunule, and absence of escutcheon. 



Some may think that the best plan would be to unite the Pitaria 

 and Amiantis groups, and to make only one genus of them, but this 

 would obscure the fact that tliere are two essentially distinct groups 

 linked together by a few intermediate forms. Moreover, I regard 

 the complete separation of the left posterior cardinal and its extension 

 across the hinge-plate as a character of much importance, because it 

 links Pitaria with Dosinia, and suggests that the latter has been 

 evolved from the former. 



There is also another point of difference between the typical 

 Pitaria ticmens and the numerous species which have a free right 

 posterior cardinal ; for in the former the cardinals of the right valve 

 are separate from one another, like those of Callista, while in the 

 latter the anterior and posterior cardinals are more or less united at 

 the top to form an arch over the median tooth. This is markedly the 

 case in the species citrina, pellucida, and suhpellucida, the arch in 

 these being really as complete as it is in the shells whicli Dr. Dall 

 separated in 1902 under the name of Af/riopoma. In the species Iteta, 

 obliquata, and injtata, the connecting bridge is lower and slighter, and 



^ Proc. U.S. Nat. Museum, vol. xxvi, p. 388. 



