NATURAL SCIENCES OF PHILADELPHIA. 121 



is another strong one, and still beyond, a very feeble one. None of the shejl 

 being preserved, no revelations are made of the minute structure. 



Length, -17 (100) ; breadth, -15 (88) ; thickness, -09 (53). 



Collected by A. Winehell, at Rockford, Indiana. 



Tbe straight beak of the ventral valve, and the general aspect of the shell, 

 render the above generic reference unsatisfactory. Externally it seems to 

 have some relations with Trematospira and Leptoccelia, of Hall, while it still 

 more strikingly resembles Spififer Buchianus, de Kow, (Anim. Foss. pi. xv. 

 bis fig. 3, and xix. fig. 6 ;) but until its internal characters are known, I leave 

 it where it stands. 



Rhyxchonella heteropsis, n. sp. Shell small, varying from sectoriform to 

 transversely elliptic, with moderately projecting be*k ; very young speci- 

 mens in tbe shape of a barley-corn. Plications sharp, ranging in number 

 from ten to twenty ; of which three generally (sometimes two or four,) occupy 

 the sinus of the ventral valve. This valve has a moderately sharp beak, 

 turned back in an angle of 45 with the plane of the shell, and slit (in the 

 cast) from the apex to the hinge ; sinus deep toward the front of the mature 

 shell, wanting in the young one ; the plications on each side of the sinus vari- 

 able ; four in those with two plications in the stnus, six, seven or eight in 

 those with three, and five in those with four, making the whole number of 

 plications ten to nineteen. These lateral plications are bent backwards in 

 approaching the margin. Greatest prominence of ventral valve near the beak. 

 Dorsal valve more ventricose than the ventral, most prominent at the ante- 

 rior margin ; mesial fold much less marked than the sinus opposite, consist- 

 ing of two, three, four or five plications, elevated at their extremities some- 

 what above the lateral plications, the remotest of which exhibit a strong 

 downward curvature -Beak of this valve concealed beneath that of its fel- 

 low. 



Length, 38 (90) ; breadth, -42 (100) ; thickness of both valves, -28 (67). 



From one of the calcareous beds, "No. 4," of the yellow sandstone, Bur- 

 lington. " White Collection" of the University of Michigan. Also near Ham- 

 burg, Illinois, and at Weymouth, Medina county, Ohio. Whittlesey's Collec- 

 tion. 



I had hoped that these varying forms could be brought under one of the 

 numerous species already described from this group. It is a much smaller 

 shell, with more abrupt sinus than R. pustulosa, White, from the same loca- 

 lity. It is about the size of R. cameriftrn, Win., from Pt. aux Barques, but, 

 besides wanting the long dental and median plates of that species, the sinus 

 and fold are much more strongly marked, and the transverse diameter is re- 

 latively greater, giving the rostral region less relative prominence ; and the 

 mean number of plications is considerably less. In the rostral region it dif- 

 fers from R. Sugeriana, Win., in the same manner, besides being a smaller 

 shell with shallower sinus. 



Rhyxchoxella persinuata, n. sp. Shell of medium size, transversely oval, 

 with abbreviated rostral extension. Cardinal slopes nearly straight, sides 

 rounded, front straight. Ventral valve depressed, with about twenty straight 

 plications, of which eight occupy the broad and rather shallow sinus. Ante- 

 jior margin of valve abruptly deflected. Dental lamellae extending nearly 

 one-third the length of the valve. The beak of this valve projects nearly in 

 the plane of the shell, and the lateral portions of the valve are continued, 

 without convexity, to the borders, thus giving this valve a peculiarly flatten- 

 ed surface the broad sinus forming a similar plane lying at a lower level. 



Transverse diameter, -67 (100); length, '52 (77); thickness of ventral 

 valve, -16 (24). 



Burlington, Iowa, in the yellow sandstone. "White Collection ". of the 

 University of Michigan. 



1865.] 



