SSS PAL.BOXTOLOGT OF NEW- YORK. 



The epecimcns originally referred to this species occur in the Genesee slate, 

 and are more or less compressed or otherwise crushed, so that the real form and 

 pr()i)ortions cannot well be determined. The shell in this rock has been extremely 

 thin, and the incipient plications outside of the mesial fold and sinus are usually 

 almost obliterated by the pressure which has flattened the shell. In the compari- 

 son of a.considerable number of specimens from the black shale with those of the 

 bluish shales of the Hamilton group below, I am somewhat doubtful as to the 

 propriety of separating the L. muUicosta from this species, since in that one there 

 is a great variation in the strength of the lateral plications, as well as in the space 

 towards the cardino-lateral margins where the}' are more or les« obsolescent. 



The cast of the interior shows an elongate narrowly ovate muscular area in the 

 ventral valve, and a strong septum in the dorsal valve, which is divided above and 

 margined on each side l)y the muscular imprint. The entire interior of the shell 

 or cast, when preserved, is l)eautifully marked by the ramifying vascular impres- 

 sions which enclose the papillose ovarian spaces. 



The illustrations show the characteristic features of the species, as occurring 

 in the Genesee slate. The ordinary size of specimens is from three-fourths to one 

 inch in length, with a width nearly the same or a little greater. One specimen 

 measures more than an inch and a quarter in length and an inch and a half in 

 width. 



Geological foi-mation and localities. This shell occurs in the Genesee slate at 

 Bigstream point and other places on Seneca lake ; near Ogden's ferry on Cayuga 

 lake, and at other places thence to Ludlowville ; below Ithaca, and elsewhere iu 

 the region of Seneca and Cayuga lakes. 



Leioiliynelius multicosta. 



PLATE LVI. 



Ltiorhynchut mvlticosta : Hall, Thirteenth Report on the State Cabinet, p 85. 1860. 

 HKynchonella f iaura : Billikos, Canadian Journal, No. xxvii, p. 273. 18(50. 

 CoinpnTe Leiorhynchut quadricoitata (loc. cit.), nt sup. 



Shell ovate, with the length and greatest width nearly equal (in com- 

 pressed specimens the width is often greater than the length) ; sides 

 gently curving from the beak for about two-thirds the length, thence 

 curving abruptly to the front, which is truncate in the middle for the 

 width of the sinus. 



Ventral valve gibbous on the umbo, curving to the sides, becoming 

 gradually depressed in the centre and forming a sinus which is broad 



