FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 53 



This genus included for a while a large number of related species, but there 

 existed differences which compelled the division into several genera, which 

 Prof. Hall and Mr. Billings established. Prof. Hall gives the characteristics 

 of Pentamerus as follows : Shells having rotund or gibbous forms, with the 

 ventral valve prominent in the middle, and the dorsal valve flattened or de- 

 pressed towards the front ; lamellae of the dorsal valve distinct, as P. knighti, 

 P. galeatus and P. pseudogaleatus. Or the form is elongate, with the valves 

 sub-equally convex, lobed or sub-sinuate ; internal structure like that of P. 

 knighti, as P. oblongus and P. lens, etc. 



Pentamerus complanatus. K. sr. 



Plate XXVII., figures 14, 15 and 16. 



Pentamerus nysius, var. tenicosta, Hall. 27th Rep. of N. Y. State Museum, plate 10, figures 1, 2 and 3. 



Prof. Hall described, in the 24th Report on the N. Y. State Museum, pages 

 184 and 185, a new species, the Pentamerus nysius, and distinguished of the 

 same the two varieties : Pent, nysius, var. crassicosta and Pent, nysius, var. 

 tenuicosta. In the 27th Report on plate 10, he figured two different shells, one 

 sub-figures 1, 2 and 3, and the other sub-figures 4, 5, 6 and 7, and placed both 

 in his new species. The great diversity in their form Prof. Hall regards as 

 the results of age, but not of specific importance. Only on account of the 

 different size of their radii, he placed them into different varieties. Now I 

 have before me more than fifty specimens, and find among them both forms, 

 each one represented by specimens, from very young or small to very old or 

 large, showing that the age did not cause the diversity of form, as shown in 

 Prof. Hall's two shells. This difference in shape I consider of sufficient impor- 

 tance to base upon it a new species, and the only question is now, which of the 

 two forms shall retain the original name, and which shall form the new species. 

 The elongated and very gibbous shells represented by figures 4, 5, 6 and 7, show 

 a marked difference in the size of their costae, which necessitates the separa- 

 tion of the species into two varieties, to be designated as crassicostus and 

 tenuicostus ; I, therefore, retain the original name for this form, while the 

 shell, represented by the figures 1, 2 and 3, forms the new species with the fol- 

 lowing description : Shell of medium size, broadly triangular, with little 

 gibbosity ; both valves almost equal in size, form and convexity. The regular 

 curve of the front margin is only slightly disturbed by a faint mesial exten- 

 sion; the lateral margins are nearly straight from the apex down to two- 

 thirds of the length, and slightly curved towards the front. None of the 

 shells show the least indication of a mesial depression or elevation ; surface 

 marked by fine radii ; ventral valve a little more convex than the other, being 

 depressed convex ; greatest convexity below the umbo, from where it slopes 



