20 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



3. The homologies of the liiiige elements in Chamidae and tlie reasons for 

 constitnting the new genus Pseudochama. 



On the assumption tliat the juvenile shell in either group of the recent Cha- 

 midae agrees essentially with either of the types described above, we are dealing 

 with two separate lines of evolution: that of the »inverse» Chamas and that of the 

 »normal» ones. In the latter forms we trace, even from the earliest stages, the chief 

 arrangement of the persistent dentition, the formula of the juvenile as well as that 

 of the adult stage being expressed, as stated above, in this manner: 



Left valve: I 2 4 b 61> II 



Right valve: 3 a 1 3 b 5 b I 



In the »inverse» forms, on the contrary, a metamorphosis takes place, resulting 

 in an obliteration of the primary median tooth of the left valve, which is contained 

 among the rugations of the adult hinge designated by r above, as well as a change 

 of the primary median right tooth, which becomes united to the anterior one and 

 is only occasionally separated from it by a furrow corresponding to one of the left 

 rugations (r). Lastly a reduction of LP II takes place, compensated by a develop- 

 ment of a new tooth element, the marginal tooth. 



The development of this type consequently fully confirms the view expressed 

 above as to the difference between the hinge of the »inverse» forms and that of the 

 »normal» ones. Even for the juvenile stage it is possible to establish nearly all the 

 same elements as in the adult, but their nature as distinct teeth is more obvious 

 in the young shell. This circumstance makes it evident that a comparison of both 

 the types in order to homologize the teeth of the »inverse» forms will be much more 

 easily performed on the basis of the juvenile characters than on those of the full- 

 grown individual. 



Leaving out of view the differences in size etc. that arise during the increase, 

 we ha ve found in the young stage of Echinochama and the remaining »inverse» forms 

 a dentition of the same elements as in the adult shell, thus being given by the 

 formula 



Left valve: 



Right valve: 



X II 



YA Yp 



X is the small posterior, x the small anterior and r the large median cardinal ; 

 YA and Yp are of equal strength, and y is of inconsiderable size. 



By designating the cardinal teeth according to the principles of Bernard we 

 have to reckon with three alternatives: 



,, 4a 2 4 b „s 2 4 b 6 b ,s 2 a 2 b 4 b 



i; — — — — ; -i] — 5-; — =— r — ' ö ) 



3a3b5b 3b 5b 7b 1 3b 5b 



