70 PALAEONTOLOGY OF THE UPPER MISSOURI. 



localities or particular positions. Where the conditions were favorable, the shells 

 attained a larger size, grew more symmetrically, and present the normal form of a 

 true Gryphcea ; but where exposed to the action of waves or too strong currents, 

 they were probably more firmly attached, are smaller, more irregular in form, and 

 have the umbo sometimes partly, sometimes entirely obliterated by the large scar 

 of adhesion, which in a few extreme cases occupies the whole lower surface of the 

 under valve. 



From Quenstedt's figures and description of Gryphcea calceola, it is evident he 

 found it presenting precisely similar variations, or at any rate, that he found a similar 

 gradation of forms that he refers to the one species, Gryplvoea calceola. Whether 

 or not our shells really belong to the same species as those figured by Quenstedt, 

 or to a closely allied representative form, it is not easy to determine, without an 

 extensive series of specimens for comparison from the American and European 

 localities. In the absence of such a series we have referred our shells provisionally 

 to Quenstedt's species. 



Locality and position. The forms represented by the foregoing cuts, A, B, C,D,E, 

 are from the Jurassic beds of Wind River Mountains, in the southern part of Dakota 

 Territory. Some smaller, but similar specimens with other less regular forms, were 

 found in lied Buttes, further east in the same Territory; also at Big Horn Mountains. 

 The specimens figured on PI. Ill, are from the same position at the southwestern 

 base of the Black Hills, Dacota Territory. 



FAMILY PECTINIDJE. (See page 48.) 



SUBFAMILY PECTININjE. (See page 48.) 



Genus CAMPTONECTES, AGASSIZ. 



Synon.Pecten (sp.), SOWERDT, Min. Conch. Ill, ISIS, 3. ROEMEK, Die Vert, des Nord. Kreid. p. 50. D'ORBIGNY, 



Pal. Franc. Ill, p. 592, and of various others (not MDLLER). 



Camptonectes, AGASSIZ, MSS. MEEK, Smitbsouian Catalogue Jurassic Fossils of North America. 

 Klym. af*irTijf, curved ; mxi-n;, a swimmer. 

 Examp Pecten lens, SOWEKBY. 



Shell thin, subequivalve, lenticular, closed ; hinge generally short, straight, 

 edentulous ; ears compressed, anterior one of the right valve separated from the 

 margin below by a well defined, often deep, byssal sinus. Surface ornamented 

 with radiating, impressed striae, which curve strongly outwards in approaching 

 the lateral margins, and often present a punctate appearance produced by the 

 crossing of regular concentric striae. Muscular impressions faint, apparently as in 

 Pecten. (Animal unknown.) 



Prof. Agassiz proposes this genus for the reception of such species as Pecten lens, 

 and P. obscuntfi, Sowerby ; P. striatoyunctatus, Roemer, P. virgatm, Neilson, &c. 

 It will probably be also found to include several nearly smooth or concentrically 

 striated Jurassic and Cretaceous forms, since wo find faint traces of curved, radiating 



