OCULAR AND GENITAL PLATES. 97 



on the exterior from what they do on the interior of the test. The madreporite shows similar 

 differential characters. On the exterior there are numerous fine pores, but on the interior only 

 one large madreporic pore as in the young, a primitive character common to the whole family. 

 Since typically in the Mediterranean form all oculars are insert, a specimen with any less must 

 be considered an arrested variant. As such variants, 11% have all oculars exsert like the 

 dominant character in the West Indian form, 4 % have V only insert, 4 % have V and I, the 

 biviiun, 10% have V, I, IV, and 14% have V, I, IV, III insert. Of aberrants there are five 

 specimens, 4%. Of these, one has V, IV only insert, two have V, I, III, and two have 

 V, IV, III insert. 



Cidaris affinis is an important species in these studies because it shows strongly the char- 

 acter of V, I, IV, III insert when four plates meet the periproct, instead of V, I, IV, II. The 

 same is probably true of a number of species in this order. Messrs. Agassiz and Clark (1907, 

 p. 40) in their Acanthocidaris hastigera give V, I, IV, III insert as the species character, but do 

 not say how many specimens .showed it. It would be inter- ttt 



esting to know, as it is the only species of echinoid known in 

 which this is the character, as far as I am aware. 



Eucidaris metularia has typically all oculars exsert, but 



in 39 specimens, two have V, I, IV insert as a progressive 



variant. Eucidaris thouarsi has typically V, I, IV insert, but 



the observations show a wide range of characters from all 



exsert to all insert. This species is very close to Eucidaris 



tribuloides of the West Indies. In both species oculars V, I, » I 



/?/ 

 IV are typically insert, but in thouarsi, arrested variants are 



Text-fio. G1. — Eiiciilaris tribuloides 



common, progressive variants less frequent; while in /n6wi!oiides, (Laman-k). Florida. Diam. 5 mm. 

 arrested variants are rare and progressive variants common, x 12. 11. T. J. Coll., G45. Young 

 indicating that thouarsi is the more primitive of the two i"'""^'"'-^'- No genital and but few 



madreporic pores, all oculars exsert. 



closely related species. Of Eucidaris tribuloides 849 specimens 



were studied, and they present most interesting characters. In the young of a few mm. in 

 diameter (text-fig. 61) all the oculars are strongly exsert and there are few madreporic pores. 

 The full number of oculars travels in very early. In the adult typically (58%), oculars 

 V, I, IV are insert (text-fig. 66). The variations of the adult as seen in the large series 

 studied are most interesting as the variants cover nearly the whole range of characters occur- 

 ring as species features in all regular Echini known from the Lower Carboniferous to the present 

 time.^ The arrested variants are relatively rare, but progressive variants are common. Of 

 arrested variants, in 0.6% all oculars are exsert (text-fig. 62), as in the young and the adult 



' The only known exceptions are Gymnechinus and Slrnngijhicentmlus gibbosus, in both of which this area is peculiarly 

 modified ared typically has an aberrant arrangement of oculars (te.xt-tigs. 154—157, p. 145, and 177-179, ]). 165). 



