200 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



ORTHOCIDARIS. 



Orthocidaris Cotteau, 1862, Pal. Franc. Terr. Cre't., 7, p. 364. 



Plate 1088, Figs. 1-6, Pal. Franc, Terr. Cret., 7, Cotteau, 1863. 



Test of moderate size, circular at ambitus, very little flattened, so that it is 

 subspheroidal ; coronal plates numerous (14 or 15) ; areolae very small, scarcely at 

 all sunken, their diameter less than one-fourth the horizontal diameter of plate at 

 ambitus, and little more than one-half its vertical height; median interambulacral 

 area very broad, covered with miliaries and not sunken; ambulacra narrow, .23 

 of interambulacra, straight; poriferous zones very narrow, not sunken; median 

 ambulacral area with about 4 vertical series of tubercles ; pores oblique, separated 

 by a low elevation. Abactinal system very small, about .25 h. d. Actinostome larger 

 than abactinal system, subpentagonal, about .33 h.d. Spines and pedicellariae 1 



This is certainly a most un-cidaroid appearing sea-urchin, the straight, narrow 

 ambulacra, the numerous small and nearly uniform miliaries, and the remarkably 

 small areolae and primary tubercles are so unlike the Cidaridae, and yet if the 

 areolae were sufficiently enlarged to merge together vertically, the resemblance to 

 Polycidaris multiceps would be quite striking. Only one species has been named, 

 inermis, from the Cretaceous of Europe. 



TRETOCIDARIS- 



Tretocidaris Mortensen, 1003, Ingolf-Exp. Ech., p. 16. 



Test moderately high but very variable (.45-.85 h. d.) ; coronal plates, 4-8; are- 

 olae little or moderately sunken, tending to merge together actinally ; median 

 interambulacral area more or less depressed, bare or covered with small tubercles, 

 sutural lines usually quite distinct ; ambulacra .20-.37 of interambulacra in width ; 

 poriferous zones more or less deeply sunken ; median ambulacral area with a double 

 series of tubercles along margin, inner much smaller; intervening space may be 

 bare, or more or less covered with scattered tubercles; pores as in Cidaris. Abac- 

 tinal system .40-55 h. d., sharply defined, circular, or pentagonal; ocular plates 

 with convex or straight outer margin, little or not at all notched by ambulacra; 

 miliary tubercles covering abactinal system more or less variable in size and some- 

 what irregularly scattered, leaving bare spaces here and there, especially along 

 margins of genital plates. Actinostome, .37-50 h. d., generally smaller than abac- 

 tinal system. Primary spines 1-3 h.d., usually more or less cylindrical or terete, 

 rarely with large and conspicuous thorns, but usually covered with longitudinal 

 series of granules, which may be very low so that spine is nearly smooth or only 

 granular, or may project sharply so that spine is prickly, or may be elevated and 

 merged together so that spine is longitudinally ribbed; actinal primaries equally 

 diverse ; secondaries flat and not peculiar. Large globiferous pedicellariae some- 

 times wanting, sometimes as in Cidaris, sometimes with a very small opening and 

 a powerful end-tooth on valves, and sometimes like small ones, which have a rather 

 large opening and usually an end-tooth. 



This genus was established by Mortensen for three recent species (bartletti, 

 annulaia, spinosa) hitherto placed in Dorocidaris but whose pedicellariae, he 



