45 



lateral teeth, these shells correspoud more nearly to Corhicula ; but the 

 striae are very much less distinct than we usually see in the latter, and 

 often very nearly obsolete (quite so in slightly-worn specimens), while 

 they are mainly broken up into little granulations. 



I also observe that, in all of our species of this type, the little pedal 

 muscular scar is detached from the anterior adductor. Having no spec- 

 imens of Corhicula at hand (here in Florida, where I am writing) for 

 examination, and nothing being said on this point in any of the descrip- 

 tions of that genus within reach, I do not know whether or not our 

 shells agree in this character with the existing species of Corhicula. 

 They certainly differ, however, decidedly in this respect from two of the 

 existing Florida species of Cyrena, now before me, in which the pedal 

 scar of each valv^e is distinctly connected with the anterior adductor. 



On the whole, although still regarding these fossil shells as being per- 

 haps more nearly allied to Corhicula than to Cyrena, I am now more 

 than ever impressed with their intermediate combination of characters, 

 and think that they might, with almost equal propriety, be ranged as a 

 subgenus under Cyrena; if so, of course the name of the species here 

 described would become Cyre7ia (Leptesthes) 'planumhona. 



Locality and position. — Two hundred miles east of Denver City, on 

 the Kansas Pacific Eailroad, where they Avere found in a shaft at a 

 'depfh of 40 feet below the surface. 



Cyeena? Holmesi, Meek. 



Shell under medium size, thin, transversely ovate or subtrigonal, 

 rather compressed ; anterior side shorter than the other, and rounded in 

 outline ; posterior moderately produced and subtruncated at the extrem- 

 ity ; basal margin transversely semi-ovate, its most prominent part being 

 autero-centrally ; beaks somewhat depressed, and placed about half-way 

 between the middle and the anterior; dorsal margins forming a rather 

 long, nearly straight, or slightly convex, gentle slope behind the beaks, 

 and declining more abruptly in front, with a distinctly sinuous outline 

 just before the beaks ; surface ornamented by numerous fine, regular, 

 sharply-defined, concentric lines. 



Length, 0.62 inch ; height, 0.54 inch ; convexity, about 0.32 inch. 

 , There are among the specimens, apparently of this species, quite a 

 variety of forms, produced, as I am inclined to believe, mainly at least, 

 by accidental distortion, though the^^ may represent several distinct spe- 

 cies. The specimen from which the foregoing description and measure- 

 ments were derived presents the appearance of not having been 

 distorted, and, as may be seen by the measurements, is decidedly longer 

 than high. Others, however, agreeing exactly in surface-markings and 

 most other characters, have the length and height nearly equal, or the 

 latter even a little greater than the former. Some of the specimens also 

 differ from that taken as the type of the species, in having the posterior 

 umbonal slopes prominent and subangular all the way from the beaks 

 to the posterior basal extremity, instead of only moderately convex. 

 As above intimated, however, all of the specimens departing decidedly 

 from the typical form show more or less indications of accidental dis- 

 tortion, and present precisely the same surface- markings as the typical 

 specimen. 



In regard to the generic relations of these shells, the specimens are 

 far from satisfactory, none of them showing the muscular or pallial im- 

 pression, or the cardinal teeth. Some of the casts, however, show that 

 ihere was a posterior lateral tooth in one or both valves, rather elongated 



