FROM THE CRETACEOUS FORMATION OF NEBRASKA. 381 



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imperfect specimen, is much thicker on the inner than on the outer side of the 

 volution ; fibres longitudinal, consisting of four or six angled, more or less flattened 

 prisms, which are crossed at regular intervals of less than their diameter by septa or 

 diaphragms, and externally marked by fine transverse striae, the whole presenting an 

 appearance like a small columnar Favosite or Chaetetes. 



This specimen differs from the species figured by D'Orbigny, in being curved not 

 exactly in the same plane, in increasing much more rapidly in size from the apex, 

 and in having the fibrous portion of the shell so thick upon the inner side of the 

 volution as to bring the sides in contact if continued a single turn. The septa are 

 also much more irregular than in the European species, those which are distinct upon 

 the back of the shell often converging so that two unite in a single one on the inner 

 side of the volution. 



The differences noticed suggest an inquiry whether the generic description of 

 Caprinella should be modified ; since it seems impossible that a shell of this character, 

 from its extreme thickness on the inner side, and from its rapidly increasing size, could 

 have formed several volutions. An examination of more perfect specimens will proba- 

 bly show the necessity of such modification, or the establishment of a new genus. 



Locality and Position. Sage Creek, Nebraska. Upper part of division No. 4 of 

 Section. 



PECTEN RIGIDA, n. sp. 

 PLATE II. FIG. 4, a, i, c. 



Shell obovate, height greater than length, very gradually narrowing towards the 

 hinge ; valves equally convex ; hinge line short ; wings minute, nearly equal, anterior 

 one truncate, posterior one pointed, striated upon the surface; left or inferior valve 

 marked by strong, concentric undulations ; superior valve smooth, or marked in the 

 exfoliated shell by faint radiating striae. Length, .19 inch ; height, .23 inch. 



The strong concentric undulations of the inferior valve are likewise conspicuous on 

 the cast, and are there crossed by radiating striae. The superior valve, which has the 

 shell partially exfoliated, shows only faint radiating striae without concentric undula- 

 tions as in the other valve. Perfect specimens may perhaps show other markings on 

 the superior valve not visible in these. 



Locality and Position. Sage Creek, Nebraska. Upper part of division No. 4 of 

 Cretaceous Strata. 



