122 EelationK of Edcdusi. 



denticles. In E. vorax, tlie lower margin of the enameled sur- 

 face of the denticle is nearly horizontal and is rounded, behind, 

 with a deep notch. In the species before us, on the contrary, 

 the enamel runs down obliquely backward to an acute point, 

 from which it sweeps upward, by a gentle curve, forming a shal- 

 low sinus, to the base of the posterior row of serrations. 



From Edestus Heinrichsii, N. & W., (Geology of Illinois, 

 Vol. IV, p. 350, PI. I, fig. 1), it differs by its greater size, more 

 curved form, more oblique denticles, and. the shorter posterior 

 point of the base of the enamel. 



From Edestus minor, Newb., (Geol. of Illinois, Vol. II, p. 84, 

 PI. IV, fig. 24, and Vol. IV, PL I, fig. 2, — wrongly named on 

 the opposite page of explanations Edestus vorax), it differs in 

 its much greater size and the far broader and less decurrent 

 denticles ; as will be seen from the figures now given, repro- 

 duced from photographs of the two last mentioned species. 



Formation and Locality. — Coal measures, Decatur. Mason 

 County, Illinois. Collected by Mr. IT. A. Wheeler, of Washing- 

 ton University, St, Louis, to whom I owe the opportunity of ex- 

 amining and desci-ibing it. 



EXPLANATION OF PLATES. 



Plate IV. 



Fig. 1. Edestus DaHsii, Woodward ; Australia. 

 " la. Transverse section of same. 

 " 2. Edestus vora.v, Leidy; Arkansas. 



Plate V. 



Fig. 1. Edestus minor, Newb. ; Park Co., Indiana. 

 " la. Section near base of a larger spine of same. 

 " 2. Edestus Heinrichsii, N. and W., Belleville, 111. 

 " 2a. First segment, young spine of same. 

 " 26. Portion of later segment forming sheath. 



Plate VI. 



Fig. 1. Edestus giganteus, Newb.; Decatur, Illinois. 

 All the figures of natural size. 



