42 ROBERT TRACY JACKSON ON ECHINI. 



drobachiensis from Truro, Massachusetts (R. T. J. Coll., 821), 32 mm. in diameter, is similar, 

 with five ambulacra, but interambulacrum and genital 5 are entirely wanting. Both speci- 

 mens have five teeth. The condition in both is similar to Plate 7, fig. 5, excepting that there 

 are two oculars covering the adjacent ambulacra V and I instead of one ocular with two pores. 

 A third specimen showing this combination is a Microcyphus maculatus from Mauritius, in the 

 Museum of Comparative Zoology. In this there are five teeth, ten primordial ambulacral 

 plates, five ambulacra and oculars, but four interambulacra and genitals. Interambulacrum 

 and genital 5 are absent, so that oculars and ambulacra V, I are in contact throughout. 



10. Five ambulacra, four interambulacra, five oculars, genitals, and teeth. — This typo like 

 the last has an interambulacrum wanting, but differs in that there are five instead of four geni- 

 tals. A Strongylocenlrotus drobachiensis from Calderwood Island, Fox Island Thoroughfare, 

 Maine, measuring 34 mm. in diameter (Plate 5, fig. 16), has five complete ambulacra but only 

 four interambulacra. Interambulacrum 1 is absent and ambulacra I and II are in contact 

 throughout their length. There are five oculars and genitals, but oculars I and II are in 

 contact and shut out genital 1 from contact with the corona. This is the only case of a genital 

 dorsal to the oculars seen excepting in Bothriocidaris archaica where it is a species character. 

 Genital 1 in this case is very small and imperforate. There are five teeth, ten auricles, and 

 ten primordial ambulacral plates. 



11. Five ambulacra ventraUy, four interambulacra throughout, four ambulacra above the 

 ventral border, four oculars and genitals, five teeth. — This peculiar structure occurs in one speci- 

 men only, an Arbacia punctulata (R. T. J. Coll., 881), kindly sent me from Johns Hopkins Uni- 

 versity through Dr. G. T. Hargitt. The specimen is 43 mm. in diameter, 31 mm. high, and 

 quite symmetrical. There are five teeth and ten primordial ambulacral plates, but only eight 

 peristomal gills. At the ventral border of the corona there are five ambulacra, but ambulacrum 

 II exists for only a distance of 2 mm. in which there are three pore-pairs. Above this point 

 the ambulacrum is entirely wanting. There are four interambulacra only, the area lying 

 above ambulacrum II being a complete area, but consisting of the left half of interambula- 

 crum 1 and the right half of interambulacrum 2 (compare Plate 7, fig. 3). Dorsally there are 

 four oculars, genitals, and periproctal plates. Ocular II has quite disappeared. 



12. Pentamerous ventraUy, four ambulacra and interambulacra dorsally, four oculars in 

 place {one ventral), five genitals. — One of the most suggestive and interesting cases observed is 

 the Toxopneustes variegatus, Plate 7, fig. 2. The specimen is full grown and has a strong bulge 

 on the abnormal side. It is 61 mm. in diameter through III, 5; 67 mm. through 4, II; and 

 39 mm. high. Dorsally in place there are five genitals and four oculars, but just below the 

 ambitus lies a fifth ocular which by some chance became separated and left behind during the 

 dorsal growth of the sea-urchin. As there are the usual five areas ventraUy, the specimen 

 is readily oriented by the position of the madreporite. Ocular II, which should be between 



