No. 89.] 83 



ventral valve strongly arcuate, and truncated by a circular perforation which is 

 completed on the inner side by the outer ends of the small deltidial plates; false 

 area small, broad triangular. Beak of the dorsal valve abruptly incurved, and 

 concealed by passing within the concavity of the area of the opposite valve. 

 Surface marked by nine strong angular elevated plications; three in the centre of 

 the dorsal valve more approximate, giving the appearance of a mesial elevation, 

 and three correspondingly depressed on the ventral valve : concentric lamellae of 

 growth at irregular distances, undulated in crossing the plications, giving a series 

 of zigzag lines. Entire surface finely granulose. Shell-structure strongly punctate. 



This species differs from every other described, in its extreme gibbosity and 

 highly elevated angular plications. 



Geological formation and locality. Shales of the Hamilton group : Western 

 New-York. 



RHYNCHOSPIRA NOBILIS ( n. s.). 

 Shell large, broadly subovoid, ventricose. Dorsal valve the more gibbous, with a 

 broad moderately elevated mesial lobe. Ventral valve with a broad mesial sinus; 

 the beak large and truncated by a large round foramen, the lower side of which 

 is bounded by the summits of the deltidial plates : margins of the valve sub- 

 ulate a little below the beak. Snrface marked by numerous angular elevated 

 plications; which are sharply crenulated on the summits; the sides and inter- 

 mediate spaces finely and evenly striated. In the mesial fold there are from nine 

 to eleven plications elevated, and a corresponding number in the sinus of the 

 opposite valve. 

 The dorsal valve shows strong crural processes extending from the hinge-line for 

 a short distance, when they become slender and flattened, and below this they 

 curve and send off a process towards the centre of the shell, as in others of the 

 genus, and similar to that of Terebratula. 



This is the largest species of Rhtxchospir a now known. It differs conspicuous- 

 ly from the other species, in the defined mesial fold and sinus, and sharply elevated 

 plications. 



Geological formation and locality. Hamilton group : Livingston and Erie 

 counties. 



RHYNCHOSPIRA LEPIDA ( n. s. ). 



Shell small, broadly suboval. Ventral valve depressed convex, regularly arched 

 from beak to base : beak prominent, pointed, slightly incurved; foramen tri- 

 angular, closed by two convex deltidial plates which are excavated on their 

 inner and upper margins, forming an elongate or oval perforation. Dorsal valve 

 the less convex, most gibbous above the centre. Surface marked by about twenty- 

 four fine simple radiating plications; five in the middle of the valves stronger, 

 distinctly elevated on the ventral valve. These stronger plications extend, giving 

 a protruding form to the front of the shell. 

 This species differs from the others of this genus, in the -flatness of the valves, 



fineness of the plications, and general form. 



Geological formation and locality. Shales of the Hamilton group : Ontario 



county. 



