KUNOL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3- 5 



most frequent case and for this reason those forms have been named normal »Chamas» 

 in contrast with the »inverse» Chamas, which are fixed by their right valve. This 

 different mode of fixation has not vet been subjected to a critical analysis and has 

 been misinterpreted, giving rise to an incorrect homologization of the two groups, 

 caused by the striking similarity which they exhibit, especially externally, but also 

 in their internal characteristics hitherto stndied, that is in the striicture of the 

 hinge. 



With regard to the hinge structure of the group Chamacea the most notable 

 investigations have been carried out by Munier-Chalmas (1882) and låter on by 

 Bernard (1895, 1897). Both endeavoured to homologize the elements of the hinge 

 with those of other lamellibranchian types. In these attempts, chiefly based on 

 studies of the alternation of teeth in the adult stage, they encounter considerable 

 difficulties and are misled into homologizing in cases where superficial similarity 

 and merely analogous conditions prevail. 



Bernard, in his works on le développement et la morphologie de la coquille 

 chez les lamellibranches», distinguishes between two chief types of lamellibranchian 

 shells, based on differences in the construction of the hinge: 1) the cyrenoid type, 

 in which the median cardinal of the right valve fits in front of the median cardinal 

 of the left valve, thus being, according to Bernard's denomination, the innermost 

 tooth of the hinge or no. 1; and 2) the lucinoid type, in which the left cardinal, 

 tooth no. 2, fifts before the chief right cardinal because of the absence of tooth 1 

 within the hinge. 



In the case of Ghama Bernard assigns the > normal» forms of this genus, 

 which are attached by their left valve, to the lucinoid type, because the cardinal 

 tooth of the left valve corresponds to the lamella 2 of this type. In the right shell 

 the thooth 1 is represented by a small projection in front of or inside the chief 

 cardinal; the last-named shows a tendency towards bipartition into 3a and 3 b (ef. 

 fig. 1 [=fig. 22 in Bernard 1895]). 



Concerning the »inverse» Chamas (attached by the right valve), Munier- 

 Chalmas had previously drawn attention to the striking resemblance between the 

 dentition of the right (fixed) valve and that of the left (fixed) one of the »normal» 

 forms. His somewhat superficial investigation made him believe that this similarity 

 was due to a real reciprocity of the details, and thus he committed the error of 

 assuming that it was of little importance to consider the symmetrical position of 

 the valves (if left or right) by comparing and homologizing their dentition, this 

 showing, in his opinion, the same elements in analogous cases, so that the fixed valves 

 in every case (if sinistral or dextral) corresponded to each other, as did likewise the 

 free or upper ones. 



This seeming resemblance of the analogous valves caused Munier-Chalmas to 

 make an attempt to avoid the difficulty in its explanation by proposing a new 

 terminology or notation for the shells in Ghajnacea. He distinguished between valve 

 * and valve |5. As the first-named he indicated a valve with the following charac- 

 teristics: 1) an anterior cardinal socket, 2) an oblique cardinal tooth, 3) a posterior 



