INVERTEBEATES. 831 



the ambulacral plates, the concave sides of the adambulacrals, and 

 the attaching ends of the succeeding plates, so that they are bounded 

 by four plates while nearly surrounded by two. 



The oral plates form, in pairs, elongated triangles, with the acute 

 angle directed toward the center of the visceral cavity. The apices 

 are joined by a suture with the basal portions, which are partly 

 separated by a dividing furrow. A single genital pore perforates 

 each basal section of the oral plates. Dorsal side unknown. 



Locality and position; Prairie du Long c;eek, Monroe county, 

 Chester limestone. 



No. 2479, Illinois State collection of 1878. 



ORDER PERISCHCECHINID^]. 



FAMILY LEPIDECHENTCLE. 



HYBOCHINUS, n. gen. 



(Ety hubos, hump-backed; echinus, the sea urchin.) 



Test flexible subspheroidal, and consisting of five (?) ambulacral, 

 and the same number of interambulacral areas. Ambulacral areas 

 composed of numerous ranges of interlocking and overlapping plates, 

 each of which is perforated in the central part, by a single pair of 

 pores. The plates imbricate from below upward. Interambulacral 

 areas narrower, and consisting in the equatorial region of five or 

 more ranges of overlapping plates some of which disappear before 

 reaching the poles. The plates imbricate from above downward and 

 from the central range outward. Surface covered with small gran- 

 ules not large tubercles for the articulation of minute spines. 

 Jaws consisting of large, subtriangular, truncated conical pieces, 

 deeply furrowed towards the ends, and perforated in the central part. 



This genus is related in many respects to Lepidesthes, but differs 

 in the important particular of having the imbrication of the plates 

 in the opposite direction, as in the Echinotlmridce or more modern 

 family of flexible Ecliinoids. The humps and protuberances of the 

 upper end of the type specimen, which suggested the generic name, 

 are supposed to indicate the great flexibility of the test, and an ab- 

 normal development of the number of ambulacral plates. An ab- 



