116 ROBERT TRACY JACKSON ON ECHINI. 



exsert. In 30% ocular V is insert, 14% V, I, and 33% V, I, IV. Tliis last is the typical 

 feature by a small margin. This is a curious case in which the typical character falls so nearly 

 on two distinct points. It is interesting also that these two points are not structurally contig- 

 uous. A large number of observations would be interesting. There are ten aberrants, 13%, 

 which is a very high percentage in the order, and these variants are all V, IV insert, a common 

 aberrant in this genus. It quite probably may be considered an imperfect V, I, IV with I left 

 out by failure to become insert. Mr. Agassiz (1873, p. 403) says of this species, "The ocular 

 plates are in contact with the anal system." From his context this is evidently a misprint 

 for two ocular plates. In no case seen in the genus do all the oculars reach the periproct as 

 the quoted sentence implies. 



The South American species, Arbacia nigra, may be considered the most progressive of 

 the genus with typically three oculars insert. In ten young specimens a few millimeters in 

 diameter, all the oculars are exsert as in the typical condition of adult punctulata. Of adults, 

 in 246 specimens only 1 % have all oculars exsert, the typical character for several species, 

 but an extreme arrested variant in nigra. Ocular V only is insert in 22% and V, I in 12%. 

 Both of these are arrested variants for nigra while they would be progressive variants for most 

 other species of the genus. The typical character of V, I, IV insert (text-fig. 167, p. 149) 

 occurs in 57%. Aberrant variants are unusually frequent in this species, 8%, which is high 

 where a considerable number of specimens were examined. Thirteen of the nineteen variants 

 have V, IV insert, a conmion variant in the genus, as noted under spatuligera. One speci- 

 men has oculars I, IV, and one has I, V, III insert. Four of the variants have V, I, IV, 

 III insert, a sequence also noted by Clark (1907, p. 193). These are the onlj^ cases of four 

 oculars insert seen in the family; it is a rare sequence in the order, but quite common in the 

 Cidaroida. Arbacia nigra with a high percentage of three plates insert is in this character 

 the most progressive species of the genus, and it is of importance that three other characters 

 bear out the same conclusion. The auricles of the perignathic girdle quite commonly meet in 

 an arch (text-fig. 228, p. 193), a progressive character not usual in other species (text-fig. 

 227, p. 193) ; also there are more pore-pairs to an ambulacral plate than in other species, and 

 secondary spines are present, all concurrently marking it as the highest species in the genus. 



In two specimens of Coeloplewus floridanus, all oculars are exsert as in the more primitive 

 Arbacias, and in the young of the more specialized species of the same. The Arbaciidae present 

 quite a limited range of characters in regard to ocular plates. All plates exsert is the most 

 frequent species feature, or three plates may be insert. All other characters found are variants, 

 either arrested, progressive, or aberrant. Of 53 aberrants seen in the family, 40 are cases of 

 oculars V, IV insert; the other thirteen aberrants are scattering (tables, pp. 158, 164). 



Of the Camarodonta, which is the last suborder of the Centrechinoida, I have had the 

 opportunity to examine many species and large series of several. 



