78 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Tliis species seems to be nearly related to Pccten lens, of Sowerby ; but as no good 

 figures or descriptions of the right valve of that species, from the original locality, 

 have yet been published, and several distinct species have probably been confounded 

 under that name, Ave are somewhat at a loss how to point out the distinctive charac- 

 ters of our shell. Sowerby's figures and description, especially, give us no very 

 satisfactory characters for identification or comparison, and that given by Morris & 

 Lycett (Monogr. Grt. Oolite, tab. ii, fig. 1) is also apparently of a left valve only, 

 though from a better specimen than those figured by Sowerby. Compared Avith 

 this, our shell is proportionally broader, and has a shorter hinge line, as well as 

 shorter ears. From the species figured by Goldfuss, under SoAverby's name (Petref. 

 Germ, ii, tab. xci, fig. 3), it Avill be readily distinguished, by its smaller posterior 

 ear, and much deeper byssal sinus ; this latter character Avill also distinguish it from 

 the forms figured as Pecten lens, by Bronn (Leth. tab. xix, fig. 7). 



The species figured by D'Orbigny (Geol. lluss. II, tab. xlii, 1) as P. lens, re- 

 sembles that before us very nearly in form, the depth of its byssal sinus, and most 

 of its other characters, but our species may be at once distinguished by its shorter 

 obliquely truncated posterior ear. As near as can be determined from D'Orbigny's 

 figures, the form described by him seems to be more coarsely striated. 



Locality and position. — Red Buttes, and beloAV there on the north branch of 

 Platte River, Dakota Territory. Jurassic. (Type No. 680, Smithsonian Museum.) 



Camptoiiectcs ? extenuatus. 



(Plate iii, Fig. 6.) 



Pecten extenuatus. Meek & Hatden, Proceed. Acad. Nat. Sci. Pliila. May, I860, p. 184. 



Shell small, broad-ovate, or snbcircular, compressed, lenticular; ventral margin regularly rounded; hinge line 

 short (ears unknown). Surface of casts apparently only marked with concentric stri.'E. 



Antero-posterior diameter, 0.90 inch ; breadth from hinge to ventral margin, 0.98 inch ; convexity, about 0.28 inch. 



This species is rather abundant, but all the specimens Ave have seen are casts, 

 which do not shoAv the form of the ears very clearly. In one specimen, hoAvever, 

 in removing some of the matrix from the hinge, Ave thought Ave saAV traces of a 

 deep sinus under the anterior ear of the right valve. If so, the form and general 

 appearance of the shell would be that of Camptonectes, unless there may be difier- 

 ences in the surface markings. As stated above, the casts we have seen only show 

 faint traces of concentric striae ; but as they are in sandstone, it is possible there 

 may have also been fine curved radiating striae, if not as in the typical species, at 

 least faintly indicated as in Pecten cottaldinvs, D'Orbigny. 



The uniformly smaller size of this shell Avill alone distinguish it from the last, 

 even if it should be found to agree in its surface sculpturing. 



Locality and 2yos it ion. — North Platte below Red Buttes, Dakota; and southwest 

 base of Black Hills, Dakota Territory. Jurassic. (No. 680, Museum Smithsonian 

 Institution.) 



