CA K li()\ I I i: 110US A. 27 



would probably include our shell which aijrei > \. i\ closely in Conn and snrface- 

 markini;s with MIMIC lit' thosi- figured !>y Mr. Davidson. In t'unu it serins to be 

 most nearlv allied to the s|,, , 1,-s nr \ariet \ ,,/.//.///* of Phillips, hut differs in having 

 its stria' of nearly uniforin si/e. Internally it also differs from that and all tin- 

 other analogous species or varieties, of \\hich \\e ha\e seen figures, in haunt; the 

 cardinal process ,,f the dorsal \;d\e proportionally narrower, and the socket plates 

 \\idely divergent. The innscnlar and \isceral impressions of its ventral \al\e, 

 as ma\ he seen 1)\ our figures, occupy a |iroportionally larger space, extending out 

 as the\ do aliont half the distance from the hinge to the front; while in the forms 

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

 the hinge to the front. These differences, how i \c r. may not be of specific value. 



Lix-<iHii/ 'in'/ //.<s,v;,,,,. I.eaM-nworth City, Kansas, from a bed nearly on a level 

 with the Missouri Ki\er. Coal Measures. (No. 1010.) 



CLASS LAJMELLIBRANCHIATA. 



FAMILY PTERIID^E. 



(-AVICCLID^B.) 



inrquivalvo, inequilateral, composed of an inner laminated pearly 

 . and ;in (niter prismatic substance; left or upper valve always more 

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

 mure in less simmns for the passage of the by ssus. Cartilage submarginal, 

 simple, and placed in a single cavity or depression 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 Heri'mai or ArimH,,,,-, and Melinina:) arc both represented by living species 

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

 follows : 



