274 CEETAOEOIJS PELECYPODA 



in the closed shell. Another species which I have examined is Mytilicardia 

 ( Beguina) semi-orbiculata, Linn., from the Mcobars. It has the mantle again 

 open all round, with a rather large posterior anal opening ; there are three large 

 sub-equal leaflets of gills on either side; the mouth is large and the four palpi 

 small, narrow, elongated ; the foot small, compressed, with a very large byssal 

 groove and strongly developed byssus below. 



Thus this account of the animal of Cardita differs^ considerably from that given 

 by E/Ccluz, but both the species agree in all essential characters with the anatomy 

 I observed in Astarie, and I do not see, therefore, any sufficient reason as yet to 

 alter the arrangement which I have here adopted in the classification of the various 

 genera. We must, as I said, await further elucidation on this point through the 

 examination of more species. 



The AsTAUTiDJE are numerous through all the geological formations. A few 

 like Fachydoynus and Astartila are only represented in palaeozoic times. During 

 the middle mesozoic period the astabtin^ were most varied in generic types and 

 most numerous in species ; after that time their number decreases, and at the present 

 the recent Astartes are almost confined to the northern seas, while Oouldia is 

 also tropical. On the other hand, the carbitinje are mostly all tropical species ; 

 there are, at least, 80 known from present seas. They are also represented already 

 in palaoozoic beds, but their number remains comparatively small through all the 

 succeeding epochs, though sensibly increasing, until in the eocene they reach a 

 great development, but, I think, their number is somewhat smaller than that of 

 the present time. 



1. Megalomus, Hall, 1852, (Palseont. of New York, vol. ii, p. 343). Shell 

 longitudinally oblong, moderately inflated, beaks anterior, incurved, tumescent, 

 and considerably more thickened than the rest of the shell, which is rather solid ; 

 hinge with several short folds below the umbo ; muscular impressions deep, rounded, 

 the anterior smaller, and with two superimposed small impressions. Type, 

 M, Canadensis, Hall, from silurian rocks. 



The form of this shell somewhat resembles MyoconcJia^ but the thickness of 

 the shell and the incurved beaks appear to indicate a greater relation to Megalodon 

 than to that genus. I do not think that the typical species of Megalomus can be 

 identified with Cypricardites o^ Conrad. 



2. Fachydomus, Morris, 1845, (Strzelecki, New South Wales, p. 271; Dana 

 in Expl. Exp. Geol., p. 692). Shell solid, inequilateral, and sub-triangularly elon- 

 gated, posteriorly obliquely truncate, concentrically lamellated or striated, two 

 deep sub-equal anterior muscular impressions, posterior large, but less excavated ; 

 hinge with one, or two (?) cardinal teeth ; internal surface of shell rugose, liga- 

 ment strong external; pallial impression thick, entire, but truncate posteriorly 

 being parallel to the posterior margin, or nearly so; type, F. cuneatus, Sow., from 

 palaeozoic rocks of New South Wales. The hinge-teeth of this genus are insuffi- 



* Recluz says that there is only one gill on each side. 



