CRETACEOUS FOSSILS. 269 



TRIGONIA, Brug. 



T. Mooreana, Gabb. 



(T. crenulata, Roern. (not Lam.) ; Krcid. Tex., p. 51, pi. 7, fig. 6.) 

 (T. Mooreana, Gabb; Synopsis Cret. Moll., p. 176.) 



Shell elongate subtrigonal, beaks small, terminal, incurved; 

 anterior end slightly convex, rapidly sloping backwards; base 

 forming a regular curve, continuous with the anterior margin ; 

 posterior cardinal margin broadly and slightly concave ; posterior 

 end unknown. Surface ornamented by about twenty-five moder- 

 ately large and slightly sinuous ribs, very slightly crenulated, 

 with broad concave interspaces. These ribs become obsolete on 

 the border of the corselet, or cross it transversely as fine lines, 

 continuing across the central area as sharply defined, linear ribs, 

 in a transverse direction, slightly curved forwards at their ex- 

 tremities. Corselet broad, nearly flat, with a narrow border, 

 marked by a deep longitudinal groove. 

 • 



Length of a broken specimen, less a portion of the posterior end, 2.4 inches, 

 probable total length, 2.8 inches ; width, 1.5 inch, height of single valve, .9 inch. 



Not rare at Arivechi. 



The present species belongs to a large group in the genus, all of which are more 

 or less closely allied. It can be distinguished from T. crenulata, its nearest ally, 

 and to which Dr. Roemer referred it, by its more slender form, less prominent 

 base, less strongly crenulated ribs, and by the character of the corselet. In 

 Lamarck's species, that area is comparatively smaller, and is bordered by a wide 

 margin, over which the ribs cross very obliquely backwards from the beak, after- 

 wards crossing the area itself almost transversely or obliquely forward, though in 

 a less degree than on the margin. In our species, the border of the area is narrow 

 and deeply grooved, and the linear ribs are obsolete, except near the beaks, where 

 they are transverse, instead of having the marked obliquity seen in crenulata. 



It differs from T. Emoryi, Con., in being more elongate, in having a much nar- 

 rower corselet, and in other minor details. From T. Evansana, Meek, it can be 

 distinguished by its less prominent base, more retreating anterior end, the greater 

 number of its ribs, and by the entirely different direction of the lines on the 

 corselet. 



