THE PERIPROCT. 



175 



of these two, which appears to be the suranal, hes dorsal to genital 3, as in Salenia. In Acro- 

 salenia wiltoni (text-fig. 105, p. Ill) there are four large periproctal plates besides a vacant 

 space, doubtless for small anal plates as in Salenia. The largest of these four is probably the 

 suranal and lies dorsal to ocular III, as in Acrosalcnin spinosa and Peltastes. 



In Arbacia there are typically four plates in the periproct, l)ut as variants there may be 

 fewer or more than this number. Bell (1879) notes that there may be as few as three or as 

 many as ten, and Mr. Agassiz (1881, p. 57) notes that there may be more than four up to thir- 

 teen periproctal plates. I noted (1899, p. 131) that in Arbacia punctulata there may be as few 

 as three or as many as nine plates as variants from the typical four. In Arbacia the typical 

 four periproctal plates lie in such planes that two are in the antero-posterior axis III, 5, and 

 two are on each side of this axis (text-figs. 111-114, p. 115; text-figs. 107, p. 149; 202). The 



Y 



204 



205 



205a 



206 



Text-pigs. 200-20G. — Variation of periproctal plates in Arbacia with the tj'pical character in Habrocidaris. 



200. Arbacia punclulala (Lamarck). Woods Hole, Massachusetts. Diam. 25 mm. R. T. J. Coll., 727 

 Only two periproctal plates; suranal dorsal to genital 3. 



201. The same. Diam. 45 mm. R. T. J. Coll., 728. 



Diam. 47 mm. R. T. J. Coll., 729. 



Diam. 31 mm. R. T. J. Coll., 730. 



Diam. 36 mm. R. T. J. Coll., 731. 



Diam. 43 mm. R. T. J. Coll., 732. 



X3. 



202. The same. 



X 3. Only three periproctal plates. 



X 3. Four periproctal plates, the typical character (p. 1 1."").) 



X 3. Five periproctal plates; compare text-fig. 206. 



X 3. Six periproctal plates. 



X 3. Many periproctal plates. 

 Florida. Diam. 32 mm. R. T. J. Coll., 890. X 4. One large and about fifty small periproctal 

 plates. The specimen is so distorted its ocular arrangement is omitted in tabulation of the species, pp. 115, 153. 



206. Habrocidaris scutata (A. Agassiz). Santa Cruz, West Indies. Diam. 17.5 mm. Adapted from A. Agassiz and 

 Clark, 1908, Plate 54, fig. 5. X 3. Five periproctal plates; compare text-fig. 203. There is only a single madreporic 

 pore in genital 2 (compare Plate 3, fig. 14). The genital pores lie very far tlorsally (p. 172). 



203. 

 204. 

 205. 

 205a. 



The same. 

 The same. 

 The same. 

 The same. 



