FOSSILS OF THE SILURIAN AND DEVONIAN BOCKS. 55 



elevation are not at all indicated. The surface of the larger portion of both 

 valves is entirely smooth ; the basal margin shows, however, from eight to ten 

 plications of different strength and extent on each valve ; the central ones are 

 stronger and longer than the balance ; they diminish in size and length accord- 

 ing to their distance from the central point of the base. None of these plica- 

 tions extend beyond the basal third ; most of them are confined to the border. 

 An average-sized specimen measures five and one-half lines in width, four and 

 one-half lines in length, and three and one-half lines in depth. This shell 

 has some resemblance to P. sub-globosus of Meek and Worthen, from the 

 Hamilton group, described in the Illinois Report, Volume III., but it differs 

 from that species by its smaller size, less gibbosity and smaller number of 

 plications. 



Formation and Locality. Found in the Niagara rocks, east of Louisville, in almost perfect speci- 

 mens, but rather rare. * 



Pentainerus knappi. HALL. 



Plate XXVIII., figures 1, 2, 3 and 4. 



Pent, knappi, Hall. 24th Rep. N. Y. St. Museum, p. 1841872. 

 Pent, knappi, Hall. 27th Rep. N. Y. St. Museum, pi. 10, figures 10, 11 and 121875. 



Shell broadly elliptical, moderately gibbous above, compressed in front ; 

 length about one-third greater than width, somewhat obscurely trilobate ; car- 

 dinal line equal to nearly one-half the width of the shell. Dorsal valve 

 scarcely smaller than the ventral; moderately gibbous in the upper part, 

 broadly depressed-convex below the middle, and spreading at the latero-basal 

 margins. Ventral valve a little more gibbous in the part above the middle 

 than the opposite valve, and less depressed in the lower part ; the beak nar- 

 rower below than the other beak, above which it is visibly elevated, pointed 

 and only slightly arched. Prof. Hall in his description states the beak of the 

 ventral valve to be closely incurved upon the other valve ; his figures on plate 

 10 ; in his report, show it the same way ; this is a mistake, caused by the im- 

 perfect condition of the specimen from which description and figures were 

 made. At the time when I had my plates lithographed I had no good speci- 

 men of this species in my possession, and having then also no access to Dr. 

 Knapp's shell, which was absent from Louisville, I was compelled to copy Prof. 

 Hall's figures, carrying his mistake over on my plate. Since then I have ac- 

 quired several almost perfect specimens of P. knappi, which prove without 

 doubt the character of the beak as stated by me. 



Surface is covered with strong plications, which become almost obliterated 

 on the umbo, and which diverge and curve outward towards the front ; they 

 gire rounded, and bifurcate repeatedly with the growth, of the shell, so that 



