92 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



source, which we have assumed to be Diceras, than that they represent earlier off- 

 shoots from different points of the Megalodus tribe. The agreement with Megalo- 

 dontidae, however, undoubtedly speaks in favour of a genetic continuity between 

 Chamidae and Megalodontidae in the way that is maintained by the paleontologists. 

 Anatomical facts indicate that Chamidae occupy a separate position among the 

 Lamellibranchia, and this might be in accordance with the view presented here, 

 while, on the contrary, both anatomical and paleontological facts decidedly speak 

 against a close relation between Chamidae and Astartidae or Carditidae. 



There is, however, a circumstance that might be interpreted as a support 

 against the opinion deduced here as to the relation of Chamidae, namely the sculp- 

 ture of the shell. The forms of Diceras, like those of Megalodonlidae, are smooth or 

 furnished with a feeble concentric sculpture, while the Chamidae show the greatest 

 variation in their sculpture. Even if the com parison is extended to the nepionic 

 stage of the recent Chamidae, it results only in a statement of contrasts both in 

 form and ornamentation. The sharp lamellae of a Pseudochama as well as the weak 

 ones and the radiating striation of a Chama have no parallels in the adult shell 

 either of Diceras nor of Megalodus. Boehm (1891) tells us what difficulty these 

 contrasts offer for the understanding of the problems concerning the origin of 

 Chamidae. He remarks (p. 21), »dass die Embryonalschale von Echinochama arci- 

 nella nicht die mindeste äussere Ähnlichkeit weder mit Megalodon, noch mit; Proto- 

 diceras, noch mit Diceras besitzt . . . Die äusseren Formen hier und dort sind voll- 

 kommen von einander verschieden.» 



As an explanation of this peculiar state of affairs two alternatives are con- 

 ceivable. 1. Both Diceras and (perhaps also) Megalodus have nepionic stages similar 

 to those of Chamidae and differing from the full-grown shells; in this case they 

 should be traceable on the umbones when these are still in a good state of pre- 

 servation. The differences between the young Diceras arietina and the full-grown 

 one are, however, not great enough to support such a supposition, but a still younger 

 stage would perhaps enable us to find out whether a mode of development similar 

 to that of Chamidae is also characteristic for Diceras or not. 



If this is not the case there still remains the second alternative for explaining 

 this state of affairs. 2. From the shape and sculpture of the umbonal nucleus in 

 Chama and Pseudochama, which differs from those of the extra-umbonal shell and 

 is sharply defined from it, we conclude that the postlarval development is not a 

 direct one, as in Cardiidae and Veneridae for example, but proceeds indirectly and 

 is complicated by a metamorphosis, something like that of Unio with its glochidium 

 larva. This nepionic stage interjacent between the prodissoconch and the permanent 

 stage, is to be considered as a juvenile specialization or a coenogenetical larval stage, 

 which has arisen through a development produced somewhat beyond the direct line 

 of evolution. The umbonal shell, consequently, may be regarded as a character 

 proper to Chamidae without any correspondence in their earlier ancestors. 



That such a view is supported is proved by the following circumstances. The 

 nepionic shell varies considerably in size in Pseudochama. It seems to be larger in 



