494 PALiEONTOLOGICAL REPORT OF GEOLOGICAL SURVEY. 



rib, ornamented transversely by fine rounded strise, while the margins 

 of the pieces are similarly ornamented, by coarser striee, lying parallel 

 with the margin of the pieces, and terminating against the sides of the 

 rib which occupies the middle. The sides of the second primary ra- 

 dials are sonetimes closed upon the summit, nearly obliterating the 

 triangular field between the pseudambulacral fields. The mesial line is 

 always straight. The mouth seems to be situated at the centre of the 

 summit, from which proceed five minutely granulated, porous, pseud- 

 ambalacrse, terminating at the angular corners of the summit,' in the 

 notch of the second primary radials, forming a prominent ridge, divid- 

 ed, longitudinally, into four equal' parts by three indented lines, the 

 deepest of which rises within the mouth. The spaces on either side 

 of the middle suture are divided by small prominences, diverging from 

 the suture, and terminating within a circular depression, on the inner 

 margin of the outer spaces. Around the mouth, at the junction of 

 the ambulacral fields, are five rounded prominent tubercles — above the 

 ovarial opening, in some specimens, another is added, which is still more 

 prominent; from four of these tubercles -diverge four prominent ridg- 

 es, tapering from the mouth outward, one to the middle of four of the 

 straight sides, the fifth space is without a ridge, being occupied by an 

 ovate or circular (ovarial or anal) opening. The depressed, triangular 

 intervening spaces are filled with seven or more thin pieces, lying par- 

 allel to the pseudambulacral fields, articulating with the summit of the 

 second radial, and the prominent ridge lying between the pseudambu- 

 lacrse. These pieces were evidently capable of being compressed or 

 depressed ; the point at the lateral junction of the second radials is in 

 some specimens folded over toward the mouth so much as to entirely 

 obscure these triangular spaces by covering them. 



The ovarial or anal opening is always over the radial, to the right 

 of the incomplete fiist radial. 



Columnar facet small, round, or obscurely pentagonal. C. alternatus 

 differs from C. acntus and trilobdtus,. McCay, in its greater length, and 

 the rib ornamenting the second radials ; also, by the much greater del- 

 icacy, (judging from McCay's figure,) of the ridge between the am- 

 bulacrae. This species is found much below either of the species of 

 McCay. 



