No. 186.] 



39 



Genus Renssel^eria^ (Hall). 



Terebratula : Eaton, 1834-1842. 

 Atrypa ; Conrad, 1839. 

 Pentamerus : Vanuxem, Hall, 1843. 



Atrypa : VANtrxEji, Mather, Hall, 1843. 

 Meganteris : Hall, 1866 & 1867. 

 RensselcEria : Hall, 1868. 



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 1 

 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 tlieir anterior margins, extend about half the depth of the 

 cavity of the valve, when they turn abruptly towards the beak, 

 and apprpach 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 ex- 

 tension of which proceed the slender crural processes, first in a 

 direct line, and then one division of each, diverging into the centre 

 of the ventral valve, terminate in acute points. On the other side 

 the divisions extend nearly at right angles to the axis of the shell, 

 into the cavity of the dorsal valve; and thence behding abruptly 

 forward and gradually converging, terminate above the centre of the 

 shell in a thin flattened or longitudinally concave plate, which, at 

 its remote extremity, ends in an acute point, the whole being lan- 



* I have given this generic designation to commemorate the name of the late Hon. Stephen 

 Van Rensselaer, to whose 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 sciences, at a period when these pursuits were 

 little fostered in any of our institutions of learning; and if the results of the Geological 

 Survey in New-York are entitled to any pre-eminence, we are indebted to this early influence 

 more than to any other cause. 



