18 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



in the right valve is a false tooth, consisting of both the cardinals of the young 

 shell. One can, indeed, observe in some forms, as has already been mentioned, that 

 a shallow groove divides this geminons tooth into a larger anterior and a smaller 

 posterior part, each of which is further grooved in such a manner that the pre- 

 sence of the separating furrow grows difficult to establish and, in other forms, it is 

 quite impossible to detect any furrow dividing the geminous tooth in its primary 

 components. Where this is present, however, it corresponds to tvvo small ridges in 

 the left valve, situated just within the large dental hole; in these small ridges, 

 which do not differ by any peeuliar characteristics from the other rugations of the 

 pit ground, we recognize the homologon to the juvenile median left cardinal. Lastly, 

 in the young shell the posterior cardinal of the left valve has been greatly enlarged 

 so as to form the chief cardinal in the adult stage. 



That the process of development described above takes place in all the »inverse» 

 forms of Chamidae is evident from the circumstance that it is possible to range these 

 species according to the more or less complete stage of metamorphosis they represent. 

 Such an arrangement was indicated above; here we confine ourselves to establishing 

 that Echinochama and the other »inverse» forms agree in the chief characteristics 

 of the hinge and its development. 



On the other hand all true or normal» Chamas represent a formation and a 

 development of the hinge essentially different from the forms described just now. 

 Our investigation of the full-grown shell has enabled us to settle that the »normal» 

 Chamas represent a type other than the »inverse» ones. In their umbones they 

 generally show a nepionic shell of a more circular outline and a latticed sculpture; 

 and essential differences in the hinge are shown in the present treatise. 



In order to obtain an early stage for studying the juvenile characters I have 

 examined many shells in the collections of the Riksmuseum (State Museum) on which 

 Chamas were attached. I sncceeded in finding some small specimens on a shell of 

 Spondylus aculeatus collected by M' Andrew (1869) in the Gulf of Suez and probably 

 belonging to the species Ch. reflexa. In a left or attached valve (fig. 13) a single 

 strong cardinal projected from the dental plate. Behind it, just in the shell margin, 

 a rudiment of a posterior tooth appeared as well as a rudiment of a, lateral crest 

 farther back. In front of the large cardinal no trace of an anterior tooth was per- 

 ceptible. In the more advanced dextral valve (fig. 14) there were seen one strong 

 posterior cardinal (3 b) and one anterior smaller (3 a) ; and beneath the latter tooth 

 the ridge-like erect margin of the dental plate was distinct, running from the muscular 

 impression, where it was highest and terminated with a blunt end, to the inner side 

 of the subumbonal sinus, where it became lower and finally disappeared; this ridge 

 certainly represents a rudiment of tooth 1 in the adult. Behind the posterior tooth 

 a smaller projection occurs just before the ligamental furrow, and a trace of a 

 posterior lateral could be observed close behind the end of the ligamental groove. 



The dextral valve was covered with cuticular hairs, but none could be seen 

 in the left valve. 



Anthony (1905) has studied an earlier stage of a Chama sp. from the Gulf 



