134 Bulletin, Vanderbilt Marine Museum, Vol. IV 



genital plates. The actinal aperture is small, the cuts deep; the 

 actinal membrane is covered by minute, irregularly well scattered 

 plates. Three ocular plates are excluded from the anal system. 



The primary spines are short, about half an inch long or less, com- 

 paratively stout, tapered, with about 18 to 20 fine longitudinal stria- 

 tions. 



The ophicephalous pedicellariae are most numerous on the actinal 

 regions and very abundant all over the test, having a very long, 

 slender stem, and a rather small, globose head, each valve of which 

 has the distal margin broadly rounded, lightly crenulate. (See fig- 

 ure B.) The gemmiform pedicellariae are very numerous all over 

 the test, but especially so on the side walls and have the form figured. 

 Scattered among these pedicellariae it is sometimes possible to find a 

 solitary quadrivalved pedicellaria, of the same general structure, but 

 possessing four instead of three blades ; each composed of valves that 

 taper toward the distal end and bear a small hook. The tridentate 

 pedicellariae are the largest type found on this urchin but are rather 

 sparsely scattered among the gemmiform and ophicephalous pedi- 

 cellariae. The tridentates have the basal bowl of the head shallow, 

 the distal portion of each valve thin, much elongated, with the distal 

 margin lightly toothed. 



In his report on "Hawaiian and Other Pacific Echini, The 

 Cidaridae," Dr. H. L. Clark points out that the three nominal spe- 

 cies in the genus, gratilla Linne from the Indo-Pacific region, escu- 

 lentus Leske from the West Indies, and depressus A. Agassiz from the 

 west coast of Mexico, are distinguished on characters of only slight 

 importance, "and there is probably but a single species which is very 

 variable in form, proportions, tuberculation, character of spines and 

 color." The only character that proved constant in the study of the 

 extensive series of specimens at Dr. Clark's disposal was the amount 

 of plating on the buccal membrane; that of the "West Indian escu- 

 lentus with few small, scattered plates, while both the Indo-Pacific 

 and west Mexican species have the buccal membrane with thick, mod- 

 erately large plates. In the event that other students of the group 

 accept Dr. Clark's statement that there is probably but one valid 

 species, this would of course be known as gratilla Linne, 1758, which 

 antedates Leske 's esculentus, 1778, by twenty years. For the present, 

 I have followed Dr. Clark's example in retaining esculentus for the 

 West Indian form. 



