CARBONirEROUS AGE. 43 



This genus seems to be closely allied to some of the species included by Prof. 

 Agassiz in his genus Platymya — particularly to his P. tenuis. It differs, however, 

 from P. dilatata, the type of the genus Platymya., in being subcylindrical instead 

 of compressed, and in having the posterior side distinctly truncated and very widely 

 gaping. Again it differs in having the surface covered with minute granules — a 

 character not observed in any of the species referred to the genus Platymya. Prof. 

 Agassiz, to whom we showed our specimens, concurs with us in the opinion that 

 they are not congeneric with the forms upon which he founded his genus. 



From Allorisma of King, and Pleuromya, Agassiz, as well as the typical species of 

 Myacites, the species included in this group may be at once distinguished, by their 

 truncated and widely gaping posterior end, and less prominent antero-ventral region, 

 as well as by the more elevated position of the scar of the anterior muscular im- 

 pression. All the species yet known are also destitute of any traces of a shallow 

 depression extending from the beaks to the antero-ventral margin, so commonly seen 

 in Myacites, while they want the cardinal teeth generally present in that genus. 

 Again, they seem to have differed in their habits from the species generally placed 

 in Allorisma, which were probably, as suggested by Prof. King, surface-creeping 

 mollusks. At any rate, we observed in numerous instances while in Kansas, where 

 specimens of Allorisma were to be seen side by side in the same bed with the 

 typical species of the group under consideration, that the latter were, in nearly 

 every instance, found imbedded at right angles to the plane of the strata, with the 

 open posterior extremity upwards, as if in their normal position as burrowing shells ; 

 while the AUorismas were generally found lying in a horizontal position. 



From the genus Panojicea, which our Kansas shells closely resemble in form, they 

 differ in their greater thinness, faintly marked paUial line, granular surface, and 

 entirely edentulous hinge. 



This genus, as here defined, commences in the Coal Measures, and ranges up at 

 least into the Great Oolite, if not higher. It includes Gliainomya dilatata (= Mya 

 dilatata, of Phillips), and probably several other Jurassic species ; also our Panopcea 

 Cooperi, and Gliainomya Minneliaha (^^ Allorisma? Minnehaha, of SwaUow), from 

 the Upper Coal Measures of Kansas. 



Chaenoniya Icavenworthensis. 



(Plate II, Fig. 1, a, b, c.) 

 Allorisma? leavenworthensis, Meek & Hayden, Proceed. Acad. Nat. Sci. Pliila. Dec. 1858, 263. 



Shell oblong subcylindrical ; anterior side rounded, a little compressed and apparently entirely closed ; posterior 

 side long, truncated, and very widely gaping, the margins being even a little reflexed. Base nearly straight, or 

 slightly convex in outline, rounding up gradually in front, and very abruptly behind ; dorsal side concave in out- 

 line from the beaks to its elevated posterior extremity, and nearly parallel to the base. Beaks rather depressed, 

 somewhat flattened, incurved, nearly or quite touching, and located about half way between the middle and the 

 anterior end. Surface marked by fine lines of growth, and a few irregular, nearly obsolete concentric undulations, 

 which curve up abruptly behind, parallel to the truncated posterior margin ; crossing these, the radiating rows of 

 minute granules may be seen by the aid of a good lens, on well preserved specimens. 



Internal casts of this species show quite distinctly the scar of the anterior adductor muscle, which is oval, aud 

 located near the buccal margin, with its longer axis nearly at right angles to that of the shell. At its upper 

 extremity the small oval pedal scar is also well defined in both valves. The posterior muscular impression is 

 broad oval, and rather faintly marked ; from near the middle of its under side the pallial line descends with a 

 gentle forward curve, so as to form a broad, rounded, very shallow sinus. 



Length, 2.85 Inches; height, from the ventral margin to middle of the dorsal side, 1.36 inch; do., from the base 



