NATURAL SCIENCES OP PHILADELPHIA. 405 



Descriptions of Fossils from the Marshall and Huron Groups of Michigan.* 



BY ALEXANDER WINCHELL. 



Centronella, Billings. 



Centronella Julia, n. sp. Shell small, nearly circular, ranging from 

 slightly elongate to transverse, and squarely rounded ; both valves with regu- 

 lar lens-like convexity, sometimes with a gentle ridge running the length of 

 the ventral valve, and a slight sinuation near the margin of the dorsal. 

 Ventral valve with a moderate beak, circularly foraminated, turned up at 

 a right angle, covering the beak of its fellow. Area entirely wanting. Shell 

 obsoletely striate concentrically, and having a minutely punctate structure, 

 Apophysary system as follows : A delicate ribbon-like loop originates from the 

 stout blunt crura of each side of the socket valve, having its flat sides at first 

 vertical ; the two branches of the loop proceed at first in lines parallel or a 

 little convergent, and then gradually diverge, widening as they proceed, and 

 assuming an inclined position, until, approaching the front of the valve by a 

 regular curvature, the lower edge has become anterior, giving the band an 

 angle of 30 Q with the plane of the shell ; approaching the median line the 

 band rapidly widens, and the front margin is drawn forward in a long acu- 

 mination, while the inner margin is regularly concave, except that near the 

 median line it turns abruptly forward so as to meet that line at an acute angle. 

 The loop thus forms an urceolate figure on its inner margin, and on the outer 

 a somewhat oval one truncated behind and attenuately acuminate before. In 

 the median line where the two branches meet, both are suddenly deflected 

 downwards, forming a double vertical plate, not quite reaching the ventral 

 valve, the upper edge of which, when viewed from the side, is flatly roof- 

 shaped, while the lower edge describes two convexities, the greater, anterior, 

 leaving a notch between them. The surfaces of the loop and median plate 

 are covered with minute obliquely conical pustules, in some places seeming 

 to become spinulous. The casts exhibit on the ventral side a delicate im- 

 pressed line extending from the beak to the middle, and on the right and left 

 of this a fainter one ; on the dorsal side a median impression with two fainter 

 ones on the right, and two on the left the median terminating rostrally upon 

 a small pyramidal process (filling the beak of this valve) separated by a short 

 slit (made by the socket ridge) from a smaller isolated process on each side. 



Length, breadth and thickness of an average specimen : *31 (100), *29 (94) 



and 45 (48).f 



L^cility. Grindstone quarries, Pt. aux Barques, in a conglomeritic ferru- 

 ginous sandstone overlying the gritstones of the Marshall Group. Abundant. 



Spirifera, Sowerby. 



Spirifera subattenuata, Hall. Iowa Rep., p. 504, pi. 10, fig. 3. Comp. 

 Owen Rep. on Iowa, Wis., &c, pi. iii. fig. 9. 

 Our specimens agree with the figures and descriptions of Hall. 

 Locality. Light-house Pt. aux Barques, with Spirifera Huronensis. 



* For a description of the rocks of these groups see the author's Report on the Geology 

 of the Lower Peninsula of Michigan, 1860; also Silliman's Journal for May, 1862. 



Descriptions of 26 species of Cephalopoda from these two groups were published in the 

 number of Silliman's Journal just referred to; and descriptions of most of the Gastero- 

 poda and Lamellibranchiata of the present paper were sent for publication on the 1st of 

 April last, since which time further discoveries and investigations have extended my no- 

 tice of the paleontology of these interesting groups to its present limits, and I have for 

 this reason obtained permission of the editors of Silliman's Journal to offer the whole for 

 publication together, to the Phil. Acad, of Nat. Sciences. 



t The measurements in this paper are given in inches. The numbers in parenthesis 

 are the relative measurements that which is generally greatest being assumed 100. 



1862.] 28 



