14 NILS HJ. ODHNHR, STUDIES ON REOENT CHAM1DAE. 



replacing the cardinals, while the free valve has an arcuate ridge corrugated on both 

 sides which fits between those of the opposite valve. These are developed by the 

 deposition of shell substance at first on and about the original cardinals, connecting, 

 modifying, or submerging them, hut they are soon left behind by the rotation of 

 the valves and hinge. The small posterior laterals are almost always discoverable. 

 In the two millimetres stage the adductor scars are small but distinct, the pallial 

 Ii ne b road, entire, and slightly irregular; there is a distinct escutcheon but no 

 lunule.» 



It must be an error when Dall designates the figure accompanying this de- 

 scription (pl. LIII, fig. 1), as representing the »hinge of a very young Ghama pellu- 

 cida Conrad; from San Pedro, California; the valve 2. o mm long» etc. It corre- 

 sponds, as a matter of fact, in all essentials with the description of Echinochama 

 (except in the presence of an anterior right lateral received into a socket in the 

 margin of the left valve», a difference in description and figure that has certainly 

 caused the author's mistake for Ch. pellucida). The size of the stage shown in the 

 figure (2 mm) as well as its equivalve aspect differs completely from the charac- 

 teristics of a young »normal» Chama, as is evident from the researches of Anthony 

 (1905) as well as from my own observations; already at a size of less than 1 mm 

 in length a young »normal Chama has quite a different shape, as we shall find 

 below. The locality of Dall's shell supports the assumption that it belongs to 

 Echinochama califomica Dall 1902 (where this species is represented in a figure only; 

 a brief description is given in 1903), or perhaps to an »inverse» form such as Chama 

 exogyra Conrad. 



Even the interpretation of the hinge development given by Dall offers no 

 clear account of the real process which results in a final condition quite opposite 

 with regard to the »normal» Chamas, a view that we have reason to consider as 

 not maintainable. 



Under such circumstances a revision of the juvenile characters of Echinochama 

 and of their metamorphosis to the adult stage was very desirable. 



During my own investigations on the shells of Chamidae, my attention was 

 drawn to the difference in the umbonal region in some shells of »normal» and some 

 of »inverse» Chamas. For instance a comparison of Echinochama with Chama 

 gryphoides (small specimens) gave the result that the shape of the nepionic shell 

 remaining on the umbones was essentially different, being lengthenend and furnished 

 with distant ribs in the former species (fig. 5) and nearly orbicular with a latticed orna- 

 mentation consisting of concentric and radiating striae in the latter. Further com- 

 parisons have made it evident that in other species of »inverse» Chamas the nepio- 

 nic shell resembles that of Echinochama, and that most of the true Chamas show 

 agreement with Ch. gryphoides. In some cases, however, e. g. Ch. fragum (from 

 Ceylon), the umbo exhibits more distant riblets and a more lengthened form, thus 

 reminding one of an Echinochama. 



After a search among the collections of small shells from the West Indies be- 

 longing to the Svvedish State Museum collected by Dr. A. Goés in 1869 I succeeded 



