4a 1 PALJIONTOLOGY OF NEW-YORK. 



GEXUS 11e\SSEL.£RI.\* ( Hall ). 



Trrfbraltda : Eatov , 1834 - 1842. 



.Un/pa : Co.vrad, 1839. 



Ptntamtrxu : Vanuxkh, Hall, 1843. 



Mrypa : Vantxkm, MATnER, Hall, 1843. 

 Mffranltris : Uall, 185G & 1857. 

 Rensselaria : Hall, 1858. 



Generic Description. Shell inequivalved, oval, ovoid or suborbicular, 

 elongated or rarely transverse and sometimes subtriangular, generally 

 gibbous or ventricose. Valves more or less convex, without mesial fold 

 or sinus : beak prominent, acute, more or less incurved ; foramen 

 terminal, sometimes concealed, round or oval, the lower side formed 

 by two small deltidial pieces, and, in their absence, by the umbo of the 

 opposite valve ,^ and then appears triangular. Shell structure distinctly 

 punctate. 



Surface radiatingly striated or finely plicated, rarely smooth? 



Valves articulating by two somewhat widely separated teeth in the 

 ventral valve, with corresponding sockets in the dorsal valve. The 

 diverging cardinal teeth supported by strong dental plates, which, on 

 their anterior margins, extend about half the depth of the cavity of 

 the valve, when they turn abruptly towards the beak, and approach 

 each other or unite in the rostral cavity : from this point of return, 

 there is a low ridge bounding the muscular area, which is an elongate 

 more or less oval depression, in the centre of which the adductor 

 muscles occupy two small narrow scars; a more or less prominent 

 median septum extends the entire length. 

 In the dorsal valve, the dental sockets lie between the shell proper, 



and a strong, often much thickened process, from the anterior extension 



of which proceed the slender crural processes, first in a direct line, and 



• I have given this generic designation to coinmcmorato the name of the late Hon. Stephen Van Rkns- 

 8Ei,AEK. to wliose munificence we owe the early geological and agricultural surveys in the State of New- 

 York; and to whose liberality, in establishing the Rensselaer School for teaching the sciences with their 

 application to agriculture and the arts, I conceive is due the great impulse given to the study of the natural 

 ■cienceg, at a period when these pursuits were little fostered in any of our institutions of Icaruing; and if 

 the results of the Geological Survey in New-York are entitled to any pre-eminence, we are indebted to this 

 early in6uenco more than to any other cansc. 



