522 PALAEONTOLOGY OF ILLINOIS. 



depressed, somewhat incurved, and scarcely projecting beyond tbe binge 

 Jine ; area of rather more tban usual breadth for that of a dorsal valve, 

 and arched with the beak; mesial fold commencing at the beak scarcely 

 larger than one of the ribs on each side of it", but increasing gradually 

 in breadth and prominence to the front, where it is occupied by about 

 six to eight costae. which, however, coalesce into one or two at the beak. 

 Ventral valve regularly convex over the central region, and somewhat 

 compressed toward the lateral extremities; mesial sinus commencing 

 narrow and very small near the beak, and widening and deepening 

 gradually to the front, where it is occupied by about eight depressed, 

 rounded costae, which, like those on the fold of the other valve, coalesce 

 with those on each side and with each other, so as to leave but one that 

 extends quite to the beak; beak rather depressed and not projecting 

 much beyond the hinge line, arched or moderately incurved; area rather 

 narrow, and extending, with almost perfectly parallel margins, quite out 

 to the lateral extremities of the hinge, marked by the usual transverse 

 and vertical stria? ; foramen wider than high, rather large, aud extend- 

 ing close up under the rather flattened apex of the beak. 



Surface ornamented by depressed, rounded, bifurcating or trifid, more 

 or less fasciculated costae, about five of which, on each side of the mesial 

 and sinus fold, are larger than the others, and divide before reaching the 

 front so as to form as many fasciculi of two or three ribs each, the 

 furrows between which are less strongly defined than those between the 

 bundles. Toward the lateral extremities some eight or ten smaller, 

 .simple, obscure costae, that do not reach the beaks, may also be counted 

 on each side of each valve, gradually becoming obsolete near the ends. 

 Crossing the whole, fine obscure, undulating striae, and a few stronger 

 marks of growth may be observed on well preserved specimens, the 

 stria?, however, excepting near the front and lateral margins, not being 

 readily seen without the aid of a magnifier. 



Length about 1.15 inch, breadth 2.20 inches, convexity about 1.50 inch, 

 bight of area at the beak .26 inch. 



We have had specimens of this fine Spirifer under consideration for 

 a long time, and after numerous careful comparisons we have been 

 unable to identify it with any of the described species. It seems to be 

 most nearly allied to our common Coal Measure species S. cameratus, 

 of Morton, with which it agrees in the fasciculated character of its costae, 

 and in general appearance. It differs, however, in several characters 

 by which it can be readily distinguished on comparison. In the first 

 place, its larger fasciculated costce are distinctly broader, and rather 

 more depressed on the anterior slope of its valves, and proportionally 

 less numerous. The incurved apex of the beak of its ventral valves 

 is always less abruptly curved, aud much more flattened. The most 



