«30 PALEONTOLOGY OF NEW- YORK. 



remains of very fine closely arranged zigzag strise are seen crossing them near the 

 niai^ins of the valves. 



In some of its varieties, this species approaches the R. nucleolata ( Plate xxxi, 

 fig. 1 & 2), but differs conspicuously in beihg usually larger, and in its more an- 

 gular outline and much stronger plications. The beaks are also more prominent in 

 this species, and the dorsal valve is more extremely elevated near the front : there' 

 are also differences in the visceral impressions. These differences of character are 

 constant tlirough a very extensive series from the young to the old shells. 



This species belongs to the type of R. mlsoni, and may be said to be one step 

 farther removed from that species than R. nucleolata ; or, in other words, bearing 

 about the same relation to the latter species which that one does to R. mlsoni. 



Fig. 1 a - d. Young individuals of this species. 



Fig. 1 e - z. Illustrations of the varying forms of this species, from those of medium size 

 to the largest and most gibbous forms that have been observed. 



Fig. 2 a. A cast of the ventral valve of this species, showing the muscular impression 

 smaller than in li. nucleolata, Plate xxxi, fig. 2 x. 



Fig. 2 6. A cast of the dorsal valve of the same species. 



Fig. 2 c- Interior of the ventral valve,* which partially preserves the form of the muscular 

 imprint. 



Geological position and locality. Shaly limestone of the Lower Helderberg group, 

 Albany county. 



RIi>iichonella vellicata ( n. s.). 



Plate XXXIIL Fig. 1 a - p. 



Shell varying from transversely oval to subtriangular. Ventral valve 

 depressed convex-: beak somewhat prominent, depressed, closely in- 

 curved over the opposite. Dorsal valve more gibbous : beak incurved, 

 not prominent. 



SuEPACE marked by twenty-four to thirty-six plications, six or eight of 

 which are elevated in front of the dorsal valve so as to form a rather 

 distinct mesial prominence, rarely extending beyond the middle of the 

 valve. On the ventral valve, five to seven of the plications are de- 

 pressed towards the front into a more or less distinctly defined sinus, 

 and prolonged, forming a mesial projection which is elevated in the 

 front of the other valve. Near the junction of the valves in front, very 

 fine closely arranged lines of growth are visible. 



