KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59- NO 3. 89 



whether the family is diphyletic, so that either genus has its own origin, separated 

 from that of the other. Echinochama and Pseudochama have, for instance, perhaps 

 emanated from Venerwpis, while Chama is to be derived from Cardium or other 

 forms. The striking uniformity in organization of both genera is decidedly against 

 such a supposition of a diphyletic origin of the recent Chamidae. 



By means of comparative anatomical investigation we may certainly succeed 

 in finding the mutual relations between the recent Lamellibranchia, to estimate their 

 greater or less agreement with each other, to state their more or less primitive or- 

 ganization with respect to different characteristics and to group them systematically 

 according to the increasing perfection of one or other character. Such a system 

 may correspond to the general course of their evolution, but for a single form it 

 gives no information as to its origin in details. To get such an insight and to follow 

 the entire form in its development and not only its separate organs, we have re- 

 course to another means, without which it is impossible to gain any knowledge of 

 these problems, namely the paleontological facts afforded by the fossils. In scarcely 

 any other group of animals does there exist such good possibilities as in the La- 

 mellibranchs of tracing the differentiation of the recent forms during geological 

 epochs. In their hinge they possess an accurate indicator of the course of develop- 

 ment, and attention has always been paid to this attribute as a region of useful 

 markings, though great difficulties stånd in the way of stating a homologization of 

 the hinge elements and of utilizing them for comparative research. 



In nearly all manuals, paleontological as well as exclusively zoological (for 

 instance Tryon 1884, Zittel 1885, Fischer 1887, Cooke 1895, Dall 1895, Hescheler- 

 Lang 1900, Pelseneer 1906), the family Chamidae or the superfamily Chamacea com- 

 prises, besides the recent genus Chama, a lot of fossil ones as well, such as Diceras 

 and Requienia. It seems thus to be universally admitted that a close affinity exists 

 between these genera. 



In Diceras as in the recent Chamidae dextral as well as sinistral forms are 

 found, but there is the difference between the two groups that in Diceras the sinistral 

 forms are not >inverse» in respect to the dextral ones because the right valves have 

 always the hinge of a right valve, either attached or free, and likewise the left 

 ones. Being generally turned to the left they are generally compared with the in- 

 verse Chamas, from which, according to Bernard, they differ only in the absence 

 of the tooth 4 b. 



With the new views that have been applied to the Chamidae in this treatise 

 we shall now try to make a homologization with the genus Diceras, considering first 

 the sinistral forms and their agreement with Pseudochama. 



If we observe, for example, a specimen of Diceras arietina, as shown in 

 Hoernes's figure (1882 b , in pl. VII) we find in the right valve an anterior tooth 

 designated z and a posterior one, z x , the former apparently representing a tooth 1, 

 the latter a tooth 3 b. In the left valve the anterior tooth z is homologous with 

 2 and is divided into two parts, which surround a socket for the right tooth 1. 



K. Sv. Vet. Akad. Handl. Band 59. N:o 3. 12 



