84 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



dentition of both forms is entirely unknown, their inclusion among the genus Pseudo- 

 cha?na is very uncertain. 



In the tertiary epoch, however, typical members of both genera appear and 

 flourish even more than nowadays. Their geographical distribution was also wider 

 than the present one, though yet limited to the tropical and subtropical zones of 

 the world. Thus many forms are described from the tertiary system of France and 

 Belgium (by Cossmann & Peyrot 1908, 1911 and Doncieux 1911, not to speak of 

 older authors). From America Dall (1903) has recorded many new species of Chama 

 found in the tertiary beds of Florida; among them there are four species of Pseudo- 

 chama, of which P. draconis from oligocene »is a rather coramon species with a sur- 

 face recalling that of Echinochama y> (Dall 1903, p. 1399). Even of Echinochama 

 two species are knovvn in a fossil state, namely E. antiquata Dall 1903 and E. 

 arcinella, which occurs in the pliocene of Florida (Dall) as well as in S. Domingo 

 (Deshayes 1838; Cossmann & Peyrot 1911). 



Fossil Pseudochamas are recorded also from the tertiary of Europé (France and 

 Italy), e. g. P. aqultanica and P. sinistrorsa (ef. Cossmann & Peyrot 1911). 



From Australia Tåte (1887) mentions the tertiary Ch. lamellifera from Tas- 

 mania, thus a locality situated beyond the southernmost limit of their recent dis- 

 tribution. 



I have not troubled to make a closer study of the fossil forms of the genera 

 in question, since they present, in a still higher degree than the living species, un- 

 surmountable difficulties in discrimination. This task must be committed to Paleon- 

 tologists in connection with a necessary revision of the taxonomy hitherto applied 

 to the unusually polymorphous group Chamacea. 



9. On the origin of the recent Chamidae. 



The purpose of the inquiry made in the preceding chapters was to gain an 

 idea of the characters which are of a specific and a generic account for the taxonomy 

 of the recent Chamidae. We have showti that there are good reasons for esfcablishing 

 the new genus Pseudochama comprising the forms hitherto considered as »inverse», 

 because they differ from the »normal» ones (Chama) in both conchological and ana- 

 tomical respects. On the other liand we must admit that the great agreement 

 existing between Chama and Pseudochama in the sculpture of the shell and in the 

 main features of the anatomical organization is a sign of close relationship. The 

 differences in the construetion of the stomach and nephridia are shown to be in full 

 agreement with the characteristics of the shell and have consequently increased im- 

 portance as generic characters. Taken alone the anatomical discrepancies would 

 perhaps appear rather insignificant, if one did not remember what a slight variation 

 in form the nephridia are subjected to in the highest Lamellibranchia. Even this 

 general similarity of the nephridia gives a stronger emphasis to the discrepancies 

 where such exist. And that it offers, in certain cases, important grounds for generic 



