CARBONIFEROUS AGE. 27 



would probably include our shell — which agrees very closely in form and surface- 

 markings with some of those .figured by Mr. Davidson. In form it seems to be 

 most nearly allied to the species or variety radialis of Phillips, but differs in having 

 its strise of nearly uniform size. Internally it also differs from that and all the 

 other analogous species or varieties, of which we have seen figures, in having the 

 cardinal process of the dorsal valve proportionally narrower, and the socket plates 

 less widely divergent. The muscular and visceral impressions of its ventral valve, 

 as may be seen by our figures, occupy a proportionally larger space, extending out 

 as they do about half the distance from the hinge to the front; while in the forms 

 figured by Mr. Davidson, they only extend about one-third of the distance from 

 the hinge to the front. These differences, however, may not be of specific value. 



Locality and position. — Leavenworth City, Kansas, from a bed nearly on a level 

 with the Missouri River. Coal Measures. (No. 1010.) 



Class LAMELLIBRAIVCHIATA. 



Family PTEElIDiE. 



(= AVICULID^.) 



Shell ineqiiivalve, inequilateral, composed of an inner laminated pearly 

 layer, and an outer prismatic substance ; left or upper valve always more 

 convex than the other. Anterior margin of the right valve generally 

 more or less sinuous for the passage of the byssus. Cartilage submarginal, 

 simple, and placed in a single cavity or dej)ression near the beaks, or 

 divided and distributed in a series of furrows crossing the cardinal facet 

 at right angles — or, in some of the older fossil genera (if distinct at all 

 from the ligament), occupying linear furrows in the cardinal area or facet, 

 ranging more or less nearly parallel to the hinge line. Hinge with or 

 without teeth. Scar of adductor muscle large and usually sub-central ; 

 anterior muscular impression generally small and placed near the beaks, 

 sometimes moderately developed. Pallial line simple, often irregularly 

 dotted. 



Animal, in the existing typical genus, with mantle margin freely open 

 and doubly fringed ; foot small, grooved, and having the power of spinning 

 a byssus ; palpi large ; gills two on each side, crescent-shaped, free or 

 connected with each other posteriorly, and to the mantle. 



The above diagnosis is drawn up so as to include species belonging to three sub- 

 ordinate groups, the first of which, so far as known, has no living representatives, 

 and seems to be mainly confined to the Palaeozoic rocks. The other two groups 

 (the Pteriinm or AvicuUnce, and Melininoi) are both represented by living species 

 in our existing seas. These three sections or subfamilies may be characterized as 

 follows : — 



