42 PALAEONTOLOGY OF THE UPPER MISSOURI. 



from that of the typical species, the nearly terminal position and greater elevation 

 of the beaks giving it much the physiognomy of many species of Pholadomya. 

 Indeed, it only wants radiating costae to present all the external characters of that 

 genus. It is more nearly related to Pholadomya Omaliana, Koninck, than to any 

 other shell with which we are acquainted, from any part of the Carboniferous 

 system. Yet it differs in having more elevated, and rather more nearly terminal 

 beaks, as well as in the possession of an oblique sulcus in front of the umbonal 

 slope of each valve. The concentric undulations are likewise more distinct and 

 regular on its flanks, and its antero-ventral region is less prominent. 



It is probable that this shell will be found to present internal characters warrant- 

 ing its separation, either as a sub-genus, or as a distinct genus, from" Sedg- 

 wickia. If so, we would propose for the group the name Exochorhynchus (g^o^og, 

 prominent ; 'zi"y%o$, beak). It would include E. (Pholadomya) Omaliana, of 

 Koninck. 



Locality and position.. Juniata, on Big Blue River, Kansas. Coal Measures. 

 In the paper cited at the head of this description, Grasshopper Creek was, by some 

 oversight, erroneously given as the locality from which this species was obtained. 

 (Type 1021.) 



Genus CH^NOMYA, MEEK. 1 



Synon. Mya (sp.), PHILLIPS, Geol. York. I, 1835, 157 (not LINN.). 



Panopsea (sp.), D'ORBIGNY, Prodr. de Palseont. I, 1850, 273. MEEK & HAYDEN, Trans. Albany Inst. IV, 



March, 1858, 11 (not MENARD). 



Myacites (sp.), MORRIS & LYCETT, Moll. Great Oolite, 1853, 114 (not SCHLOT. ; MUNSTEH). 

 Allorisma? (sp.), SWALLOW, Trans. St. Louis Acad. I, 1858, 194. MEEK & HAYDEN, Proceed. Acad. Nat. 



Sci. Phila. Dec. 1858, 263 (not KINO, 1844). 

 Etym. x_wx, to open or gap; and Mya. 

 Type. Allorisma 1 leavenworthensis, MEEK & HAYDEN. 



Shell thin, equivalve, longitudinally oblong, subcylindrical. Anterior side 

 rounded, closed ; posterior side long, distinctly truncated, and very widely gaping, 

 or even dilated at the extremity. Beaks depressed and located in advance of 

 the middle. Surface minutely granulose, and usually provided with obscure con- 

 centric undulations, and more or less distinct lines of growth. "Cardinal margins 

 more or less inflected, as in Allorisma ; ligament apparently entirely external ; 

 hinge edentulous. Posterior muscular impressions placed near the posterior ex- 

 tremity of the dorsal margin ; scars of the anterior adductor and pedal muscles 

 connected. Pallial line with a broad shallow sinus. 



1 Since this description was placed in the hands of the printer, we have been led to suspect, from some 

 incidental allusions to the genus Anthracomya, in a lecture by Prof. Salter, published in the London 

 Geologist, that the group here described may possibly be identical with that genus. As we have not 

 yet seen the description of Anthracomya, however (the Memoir in which it was published not being 

 in the Smithsonian Library, nor any of those at Cambridge, New Haven or Philadelphia), .we are left 

 in doubt, and have concluded we would probably be less liable to err by proposing a new genus, than 

 by referring our shells to Anthracomya. Should they prove to belong to Mr. Salter's genus, how- 

 ever, we will cheerfully accept for them, the names Anthracomya leavenworthensis, A. Cooperi, and 

 A. minnehaha. 



