8 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



The animal was sectioned as well as a normal specimen for comparison. At 

 the examination of the sections it appeared that the abnormal specimen had a re- 

 gular anatomical organization. The asymmetry of the intestinal canal was quite 

 tvpical: a coecal appendage projected from the hind portion of the stomach towards 

 the left; the intestine' passed on the right side of the stomach and also traversed 

 the inferior end of the duodenum on the right side. These asymmetries, the only 

 ones which are to be observed in Pisidium, occur in all species of this genus. The 

 typical specimen showed the same organization in all respects. 



From these facts it is evident that the inversion is nothing but a mere dis- 

 placement of the hinge elements. Its origin was easy to trace in a large sample of 

 P. lilljeborgi, which was subject to considerable variation of the hinge characteristics. 



Bernard, who devoted himself to the most careful study of lamellibranchian 

 hinge types, did not know of any case of inversion of this kind. In his last paper 

 (1897, p. 504) he says: »Novis avons admis jusqu'ici que chaque den t a son indi- 

 vidnalité et nous n'avons pas vu de cas de transfert de type d'une valve å 1'autre». 



As, thus, no case of real inversity among the Lamellibranchia is known — 

 the ones just mentioned being nothing but slight dislocations of the hinge elements 

 it is probable that the occurrence of this phenomenon, if possible at all, is very 

 rare and of a teratologic nature. These facts, in every case, render untenable the 

 presumption of a sort of inversion as the fundamental of the theory of Munier- 

 Chalmas, and this circumstance invites a critical revision of the theory of the sym- 

 metric valves in Chamidae. 



This theory seemed to me to contain too many stränge and improbable ideas 

 to be absolutely accepted, especially as it gives no satisfactory explanation of the 

 process b}^ which a right valve may acquire the characteristics of a left one, but 

 also because it makes no attempts at taking into consideration the real homologies. 

 Finding no support for it, I tried to explain the dentition of both groups of Cha- 

 midae ont of the general rule that a right valve is homologous with a right one only, 

 and that all parts of it must and can be homologized only with those of a right 

 valve. 



In the most regular forms, thus in the equi valve Echinochama, I expected to 

 find the most primitive conditions, least altered by the attached mode of life. 

 Starting from this form, which had, in addition, never been subjected to a detailed 

 examination, I hoped to reveal characteristics which would show most clearly the 

 relations between both groups. 



In Echinochama arcinella (fig. 2) the following details of the hinge are ob- 

 servable: In the left valve a high tooth rises oblique to the dental plate and poste- 

 rior to the umbones. Its upper end is abruptly truncated and somewhat projecting 

 so as to form a small apex. This tooth, which on its upper side, parallel to the 

 ligamental furrow, is furnished with a row of tubercles or crenelations, is on its 

 opposite or inner side also furrowed and granulated. In front of this crest-tooth 

 there appears a deep socket, the inferior margin of which is thickened and marked 

 with a series of strong plicae running in a transverse direction across the dental 



