KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 93 



Echinochama (where it attains 2,4 mm in length and has 6 lamellae) than in other 

 forms of Pseudochama (at least P. ferruginea where it seems to vary from 1,2 mm 

 with 4 lamellae to a somewhat larger size with 5, and in P. gryphina, where it 

 attains 1,4 mm with about 4 lamellae). In Pseudochama (Eopseuma) pusilla it is 

 about 2 mm; the upper and smoother portion measures 0,8—0.9 mm and has about 

 4 rather prominent lamellae and traces of about 4 radial rows of squamulae; the 

 peripheral part has many radiating squamuliferous ridges). Furher, in Chama the 

 nepionic shell is very small (0,5 mm) and its smoother upper portion is only about 

 V* of the entire umbonal shell. In the spinosa-growp of Chama, however, the ne- 

 pionic shell is comparatively large. Hence we find that the more the characteristics 

 of the hinge or the sculpture of the shell differ from a more primitive stage (most 

 in Echinochama and the Chama spinosa-section, less in Eopseuma and the remaining 

 Chamas), or the more specialized these characters become when full-grown, the 

 larger is the smooth or only concentrically sculptured portion of its nepionic shell. 

 It seems as if smoothness or a radial sculpture were primitive in the earliest stages 

 of development; during the course of specialization a concentric ornamentation 

 appears, which ultimately occupies the whole of the nepionic shell as this becomes 

 larger. Or, to put it briefly, the nepionic shell is subject to a caenogenetic specializa- 

 tion, relative to the advanced development of the adult shell. In Eopseuma, we 

 may repeat, teeth 1 and 3 b are separated and of uniform size during life, while in 

 the Pseudochamas mentioned 3 b is still traceable as a distinct tooth separated from 

 1, but the latter has greatly exceeded 3 b in size; in Echinochama a separation of 

 these teeth is scarcely visible, and besides that the shell has become almost equi- 

 valve, which seems to be a secondarily acquired property. In Eopseuma the nepionic 

 shell strikingly recalls that of Chama, and this genus shows a closer resemblance to 

 its presumed ancestors (the sinistral Diceras) than does Pseudochama to the dextral 

 Diceras. In Chama the caenogenetic metamorphosis and the subsequent progressive 

 development has made the type less distanced from its ancestors than has been the 

 case in Pseudochama. 



It is evident that the genus Pseudochama has consequently been subject to a 

 progressive development both as regards its full-grown and its nepionic stages, while 

 Chama has remained more constant and conservative. It is perhaps even possible 

 that the concentric sculpture of the nepionic Pseudochama is an atavism, being a 

 reappearance of that of the earliest known cretaceous species such as Chama haueri 

 Zittel. If a true member of Chamidae at all, this form appears most likely to 

 belong to Pseudochama (ef. above). 



Dall (1903) is of the opinion that the genus Goossensia established by Coss- 

 mann (1885) is to be connected with Chama as »based on the nepionic young of 

 Chama*. In this case it ought to be allied to Echinochama, and some similarities 

 in sculpture and hinge construetion seem to support this view; but the differences 

 are even more important. The sculpture consists of strong irregular radiating ribs 

 (crossed by concentric ones), which offers a similarity to Chama (according to Coss- 

 mann), but also to a Cardita (except for the somewhat irregular appearance), and 



