94 WINCHELL AND MARCY ON FOSSILS FROM THE 



Length of shell, .57 inch; transverse diameter, .80; height of umbo of ventral valve, .31 

 inch. 



In its smooth exterior, and gently undulate margin, this species calls to mind Spirifer U- 

 costatits Hall ; and we were at first inclined to regard it as a variety of that species; but the 

 beak is always less prominent and less incurved, and the sinuations of the exterior are 

 much less apparent, and in some specimens entirely wanting. 



It is an abundant species at Chicago ; and in some instances one of the spires has been 

 found in a complete state of preservation, showing that it consists of seven turns conically 

 arranged. 



Spirifera radiata Sowerby. Silur. Sys. 637, pi. xii. fig. 6, and 638, pi. xxi. fig. 5. Very 

 good ventral valves of this species occur, and in one instance, a fine specimen with the 

 valves united. Another specimen exhibits the internal structure, showing that the spire is a 

 delicate hollow tube of about 14 turns, of which 11 are preserved. In these specimens it is 

 interesting to observe that the margin is distinctly plicated, as in the Dudley specimens from 

 England. According to Hall, (Pal. N. Y., II. 265,) this character is not observed in the New 

 York specimens of the species. Neither does it occur in specimens in our possession 

 from Waldron, Indiana. 



Spirifera crispa Sowe.rby. Silur. Sys. 624, pi. xii. fig. 8. Each spire has about ten turns, 

 the first of which corresponds to the sinus bounding the mesial elevation. The crus ex- 

 pands into a little plate and becomes coincident with the dental lamella of the opposite 

 valve. A little curved, barb-like branch departs from each of the crura, on the ventral 

 side, and the two perhaps meet together in the manner of Zygospira Hall, though in a 

 different position. 



PENTAMERUS Sowerby. 



Fentamerus chicagoensis W. and M. 



Plate H. figure 11. 



Shell of moderate size, the commissure, (including the hinge-line) presenting a somewhat 

 semicircular outline ; but in consequence of the prominence of the beak of the ventral 

 valve, the outline presented by a view from this side is obtusely sectoral. The ventral 

 valve is very ventricose, with the beak recurved over that of the dorsal valve. The highest 

 point of the convexity is two fifths the distance from the beak to the front margin. A nar- 

 row, rather deep and distinct sinus extends from the umbo to the anterior margin, which is 

 bounded on each side by a stout obtuse rib considerably elevated above the general contour. 

 In other words, this valve may be said to present a median ridge which is deeply divided 

 in the middle. Following each of the median ribs are three others, smaller than the two 

 middle ones, and of which the first is less developed than the other two. The last two ribs 

 only reach half way to the beak. Still nearer the right and left extremities the surface 

 seems to be plane, but near the beak are five or six radial striae on each side. No other sur- 

 face markings are visible either on the shell or the cast. 



Length from beak to anterior margin, .65 inch; greatest transverse diameter lying 

 midway between the anterior and cardinal margins .65 inch. 



The form of this species is similar to that of P. galeatus Dalman, but the extremities are 

 less rounded, and the costae are fewer and less regular. It is related to P. trisinuatus Me- 



