342 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



with P. avihigua and witli P. ohliqua, while in the more typical 

 Pitaria Parisiensis this tooth is completely, though very narrowly, 

 separated from the nymph. Tlie last-mentioned may therefore be 

 classed as a Pitarina, but probably most of the other s])ecies referred 

 to Pitaria by M. Cossmann in his Iconographie Complete of 1907 liave 

 an elongate partially free posterior cardinal like sidcataria; from liis 

 photograpliic figures tliis certainly seems to be the case with P. aria, 

 P. Lamherti, and P.fasfidiosa. 



Furthermore, I find tliat a few recent species agree with P. sulcataria 

 in having a similar left posterior cardinal which is only partially 

 free from the nymph, the upyier portion being closely appressed to 

 it. This is the case with P. rudis, Poli, P. Simpsoni, Dall,P. indecora, 

 Phil., P. varians, Hanley, P. hehrcca, Lam., and P. munda. Under these 

 circumstances I now regard Calpitaria as more closely allied to Pitaria 

 than to Callista, and consider that this group may be placed as 

 a section of the genus Pitaria; and that it includes the recent species 

 above-mentioned. 



Another small group of Eocene shells which I found difficult to 

 allocate to any recognized genus was that to which M. Cossmann 

 gave the name of Tivelina. The affinities of this little group are 

 undoubtedly with Callida and Amiantis, and not with Tivela as the 

 name implies, and in this opinion I am glad to say that M. Cossmann 

 now agrees with me. 



A fresh study of the specimens with which M. Cossmann previously 

 supplied me, and of two others which he has recently sent me, only 

 confirms my ])revious observations, but shows that tlie group might 

 be divided into two sections by the dentition of the right valve. One 

 of these would include the type, and agrees with Amiajitis in having 

 an undercut anterior cardinal and a channel leading from the anterior 

 lateral pit ; while the dentition of the other set differs little from 

 that of Callista. The first section includes T. tellinaria, T. Dixoni, 

 T. rtistica, T. humerosa, and T. gibbosula; the second would comprise 

 T. analoga, T. suhanaloga, T. sphenarium, and 2\ distatis. 



One common species, however (2'. striattda), occupies an inter- 

 mediate position in this respect, the anterior cardinal being deeph' 

 undercut, but there is no continuous channel in front of it. Furtlier, 

 there is no association of other specific differences to warrant such 

 a division into sections; on the contrary, they all agree in having 

 a short left posterior cardinal confiuent with the nymph, as in 

 Callista, a short narrow and nearh* entire right posterior tooth, and 

 a short rounded ascending pallial sinus. 



These fresh observations only confirm my previous statement that 

 " in Tivelina we seem to have a group of shells which has branched 

 off from the common ancestor of Callista and Pitaria, for in some 

 of the species the hinge resembles that of Callista and in some it 

 makes a near approach to that of Pitaria. Tivelina seems to have 

 been a plastic group, i.e. one which had a special tendency to develop 

 variations w^hile still retaining a certain general facies". I am still 

 able to agree with M. Cossmann in regarding the group as one which 

 is united by a common set of characters, and 1 am still of opinion 



