350 PALAEONTOLOGY OF ILLINOIS. 



GENUS EDESTUS, Leidy. 

 EDESTUS HEINRICHSII, N. and W. 



PL i, fig. la and 16. 



SPINE robust, one foot or more in length by two and a-half 

 inches wide, and one and a-quarter inch thick, composed of 

 dense, bony tissue, symmetrically flattened, with an ovoid 

 section below, lenticular above ; one margin nearly straight, 

 the other gently arched ; the basal end irregularly rounded 

 off; the arched border set with nine large, triangular, flat- 

 tened, doubly crenulated, enamelled denticles, each about an 

 inch in hight ; the upper half of the straight side forming a 

 sharp cutting edge. The denticles of the arched border are 

 broadly triangular in outline, rising perpendicularly from the 

 curved edge on which they rest, each three-quarters of an 

 inch in hight by one and a-quarter inch in breadth, com- 

 pressed laterally, with crenulated cutting edges. They are 

 contiguously placed, and each is embraced by the acute pro- 

 longations of the enamelled base of the superior denticle which 

 reaches back to its middle point. The spine is segmented 

 throughout, each segment bearing a denticle; the segments 

 overlapping to such a degree that the one bearing the supe- 

 rior denticle reaches two-thirds of the distance from the sum- 

 mit to the base of the spine. 



By a glance at the figures now given of this fossil, it will be seen that it is 

 genetically identical with that described by Prof. Leidy, under the name of 

 Edestus vorax, (Jour. Acad. Nat. Sciences, Phil., 2d series, vol iii, p. 159, 

 PL 15). The fragment upon which Prof. Leidy based his description was, 

 however, exceedingly imperfect, and left much in regard to the complete form, 

 as well as relations of this fossil, to conjecture. Though noting its anomalous 

 structure, Dr. Leidy was constrained to regard his specimen as the fragment of 

 a jaw of a plagiostomous fish. No other conclusion was fairly deducible from 

 the fragment which he had, or his proverbial aeuteness and knowledge of com- 

 parative anatomy would have led him to it. Yet the specimen before us, which 

 is nearly complete, exhibits features that seem to be incompatible with that 



