KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3- 15 



in finding one lett valve (I. 2,4 mm) of the free nepionic stage of Echinochama 

 (fig. 6), dredged at St. Martin, 200 fathoms, and a smaller right one of Chama ferru- 

 ginea (fig. 7) from the Virgin Island, 200 — 300 fathoms. I was able to identify 

 these valves by comparing them with adult specimens of Echinochama (fig. 5) and 

 Ch. ferruginea; they exhibited the same characteristics in their erect concentric 

 lamellae, which are, on the back part of the shell, suddenly terminated with a strong, 

 furrow-shaped spine. The same features are visible on the umbones of adult unworn 

 specimens. 



The most interesting characteristics of these shells is the morphology of their 

 hinges, as these differ greatly from those of the adult specimens. In the left valve 

 there are all the elements described by I)all (1903): a strong median tooth of a 

 triangulär shape flanked on both sides by deep pits. In front of the foremost pit 

 a narrow ridge-shaped tooth of about the same length as the median one appears. 

 Behind the posterior pit a callous thickening of the margin extends backwards as 

 far as the ligamental notch. Behind the ligament, on the middle of the dental plate, 

 a short lateral tooth is visible, with a concavity on its inner side for a corresponding 

 right lateral. 



In the right valve of Ch. ferruginea two teeth are to be found, both situated 

 somewhat in front of the diagonal line from the umbo to the back part of the shell. 

 The anterior tooth is parallel to the shell margin; its apex projects almost per- 

 pendicularly from a long base. The second tooth lies across the dental plate, it is 

 stronger, shorter but thicker. The two teeth are separated by a deep triangulär pit 

 evidently adopted for the reception of a median cardinal of the left valve. Behind 

 the second tooth another hole follows receiving the long posterior cardinal of the 

 left shell. Behind it a further tooth-like element is perceptible as a thickening close 

 beneath the ligamental nymph. Lastly, behind the ligamental notch, there is to be 

 seen a long lateral tooth accompanied by a furrow beneath the shell margin corre- 

 sponding to a lateral tooth of the left valve. 



It will easily be seen that these two valves complete each other, as the right 

 hinge elements fit between the left ones and vice versa. There is, consequently, ti 

 complete concordance of the hinges as well as the exteriör sculpture of the nepionic 

 shells in Echinochama and Chama ferruginea. 



In what manner the conditions just described change during the subseqnent 

 metamorphosis to the adult stage, would be of the greatest importance to know, 

 but nothing has been done so far to study this development. I succeeded, however, 

 in finding a small specimen (2,5 mm in breadth) of an »inverse» form (showing the 

 characteristics of Ch. ferruginea) attached to a shell of Spondylus americanus from 

 the West Indies, and this specimen was suitable for studying the hinge construction 

 in both valves (fig. 8). They exhibited the following conditions. 



Beginning with the left valve (fig. 8, left) we find in it, just below the umbo, a 

 trigonal cardinal tooth evidently homologous with the left cardinal of the nepionic 

 Echinochama. In front of it and separated by a pit a lengthened crest represents 

 the anterior cardinal of Echinochama, and behind the median tooth, also separated 



