aB'i PALAEONTOLOGY OF NEW-YORK. 



Aml)Oca>Ha prii>uiiiboiia. 



PLATE XLIV. 



OrthU pmumbona : Uall, Tenth Report on the State Cabinet, p. 167. 1857. 



Amboealia preeumbona : Hall, Thirteenth Report on the State Cabinet cited, p. 71. 1860. 



Shell obliquely semiglobose or ovoid, the hinge-line less than the width 



of the shell ; cardinal extremities rounded : surface without plication?. 



Vextral valve extremely convex or ventricose, with a shallow impressed 



line which has nearly the character of a narrow shallow sinus : beak 



large and incurved; area.of moderate dimensions, proportionally high, 



with the lateral margins not defined; fissure partially covered by an 



arching pseudo-deltidium. 



Dorsal valve moderately and evenly convex, usually without mesial fold 



or sinus, the cardinal extremities rounded. Sometimes there is a narrow 



impressed line down the centre of the valve. 



Surface essentially smooth, or marked only by concentric stria3, Avhich 



are sometimes crowded into imbricating folds. In rare examples, I have 



observed towards the margin a kind of punctate marking, which does 



not belong to the texture of the shell, and may indicate the bases of 



seta ; but the evidence is unsatisfactory. 



Little is known of the interior of the valves. The cast of the ventral 

 valve has a large often pustulose muscular area, margined by a thick- 

 ening of the shell ; without evidence of dental plates. The dorsal valve 

 has a bilobed cardinal process, with the bases of the crura extended along 

 the inner surface of the valve. 



Ill many specimens of this shell the length and width are ncailj^ equal, the 

 dimeasions being about three-fourths of an inch. In some individuals there is an 

 extreme elongation of the ventral valve, Jis shown in figure 5 a of Plate xliv. Its 

 greater size and absence of mesial sinus, and proportionally shorter hinge-line, 

 distinguish this species from Ainhocoeh'a umbonata. 



Geological fonruition and localities. This species occurs in the soft shales of the 

 Hamilton group, on the.shores of Seneca and Cayuga lakes, and upon Canandaigua 

 lake. Though sometimes abundant in certain layers in a few localities, it is far from 

 having the wide geographical range of the Amhocoelia umbonata. 



