90 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



Behind the large socket for 3 b there follows a tooth z±, which is evidently homo- 

 logous with LP II. 



A direct comparison based on the superficial similarity of the species just 

 described and a Pseudochama would give very incorrect results, as the anterior right 

 »tooth» of the latter is a composition of the original teeth 1 and 3 b, as stated in 

 this treatise. Consequently it does not correspond to the anterior one of Diceras 

 but to both teeth of the right valve of Diceras. 



On the other hand a comparison of the nepionic stage of Pseudochama with 

 Diceras arietina shows a better agreement of both. And if we change the full- 

 grown Diceras for a younger stage, as shown by Hoéknes (1882 b , fig. 9) the homo- 

 logy prevailing in the respective hinges is more striking because 3 b is relatively 

 smaller and of about the same size as 1. It seems not only as if in the left valve 

 of this stage 2 a and 2 b were more distinctly separated than låter on, but also as 

 if 4 b were present, fitting in the furrow behind 3 b. No distinct trace of posterior 

 laterals are to be seen in the figure, but the presence of them in the adult is a 

 criterium of their existence, probably in a more obvious form, also in the young 

 shell. 



As a consequence of the above arguments we thus note a striking similarity 

 betvveen Pseudodiama and the sinistral Diceras, especially in its young stage. 



Passing on to Chama we shall now try to find ont where its origin is to be 

 sought for. Evidently Chama cannot be compared with Diceras arietina either as 

 young nor as adult, on account of its entire tooth 2 and the rudimentary tooth 1. 

 If we take, on the other hand, a dextral Diceras (shown by Hoernes 1882 in pl. 

 VIII, fig. 9) we find at the first glance a striking agreement between this form 

 and a very young stage of Chama such as is already described in Chapter 2 of this 

 work. 



Tooth 1 is present in the Diceras in question as a somewhat ridge-shaped 

 projection extending backwards from the anterior muscular scar. Further 3 a is 

 distinct as the continuation of 3 b towards the front. A right posterior lateral is 

 also well developed — in short we find an arrangement quite homologous to that 

 described above for the early stage of Chama. 



As a result of the above comparisons it may be repeated that Pseudochama 

 shows a close relation to the sinistral and Chama to the dextral forms of Diceras. 



That the recent Chamidae and the genus Diceras are genealogically akin is 

 evident also from the fact that they are much alike externally. Stoliczka, in his 

 important work on the Indian cretaceous Pelecypoda (p. 231) states this in the 

 words quoted above to the effect that the cretaceous species partly resemble in form 

 Diceras and Requienia. 



A revision of the known fossils described as Chama is necessary before we 

 shall be able to judge the geological origin of the genera in question with any 

 certainty. 



In the work by Hoernes quoted above (of 1882) this author gives an explana- 

 tion of his view, which has been accepted by many subsequent paleontologists, about 



