TO JUNCTION OF GRAND AND GREEN RIVERS. 139 



Spirifeb (Trigonotreta ?) Texanus, Meek. 



Plate III, Jigs. 5, 5», and 5 b . 

 8yir\far ( Tri,j.»winia.') Texanus, Meek (1871), Proceed. Acad. Nat. Sol. Phila., xviii, 179. 



Shell scarcely attaining a medium size, very gibbous in the adult, varying from 

 subquadrate or subglobose to longitudinally-subovate, the widest part being generally 



in advance of the middle, and the length greater than the breadth ; hinge-line short, 

 or in young individuals scarcely equaling the breadth of the valves, and in the adult 

 often proportionally very decidedly shorter, sometimes obtusely angular at the extrem- 

 ities, while in the more gibbous individuals its extremities do not project beyond the 

 lateral slopes; anterior margin often somewhat emarginate in the middle. Dorsal 

 valve truncato-suborbicular or subquadrate, and moderately convex ; beak incurved, 

 with a narrow area, but not prominent; mesial elevation commencing- as a small simple 

 plication at the beak, but rapidly widening and becoming more prominent and angular, 

 with on each side several small costse, which divide so as to form altogether twelve to 

 sixteen at the front; lateral slopes having at the beak each two or three plications or 

 cosno, which farther forward divide so as to form as many fascicles of three each, or 

 about nine on each side of the mesial fold. Ventral valve more gibbous than the other, 

 and more strongly arched than the other from the beak to the front; beak very promi- 

 nent in the adult, always strongly incurved over the hinge; cardinal area moderate, 

 well defined, and extending to the extremities of the hinge without narrowing strongly 

 laterally, rather distinctly arched with the beak; foramen slightly wider at the hinge 

 than high, open nearly or quite to the apex, and provided with a distinct linear mar- 

 ginal furrow on each side; mesial sinus angular, and commencing very small at the 

 beak, and widening and deepening rapidly to the front, where it terminates in a 

 strongly-curved triangular projection fitting into a corresponding sinus in the margin 

 of the other valve ; surface as in the dorsal valve, the costse within the sinus being 

 smaller than those on the lateral slopes, and numbering about sixteen to twenty; fine, 

 rather obscure, undulating stria', and, near the front and lateral margins, a few stronger 

 marks of growth traverse the valves, parallel to the fvoc margins ; while numerous 

 small, rather scattering, but regularly-disposed granules, apparently the remaining 

 bases of minute spines, may be seen over the whole surface of well-preserved speci- 

 mens, which also sometimes show traces of very fine radiating stria 1 . 



Length of one of the largest specimens seen, 1 inch; breadth of same, about 0.98 

 inch; convexity, ().!)() inch ; length of hinge, 0.72 inch. Smaller specimens are propor- 

 tionally less gibbous, shorter, wider, and provided with a proportionally somewhat 

 longer hinge. 



Small specimens of this species resemble a little the more gibbous varieties of 

 S. cameratics, with a very short hinge ; but, in the adult, the valves become propor- 

 tionally more elongated anteriorly, and more gibbous, and the beak of the ventral 

 valve is so narrow, so much produced and incurved, as to present the appearance of 

 a Pentamerus. Its distinctly-granulated and minutely-striated surface, however, will 

 always serve to distinguish, even the specimens with the most extended hinge-line, 

 from the most gibbous varieties of S. camcratxs, with a short hinge. Its granular sur- 



