Bd. VI: 4) 



THE ECHINOIDEA. 



27 



toujours déprimé» accordingly does not always hold good; the fact that all DE 

 Loriol's specimens came from a single locality may account for their uniformity. The 

 outline of the test at the ambitus is generally angular, the ainbulacra forming the 

 prominent edges; sometimes, however, the interambulacra form the edges, and other 

 specimens may be quite round. 



The tubcrculation is upon the whole fairly constant. All the specimens have a 

 broad bare interambulacral space on the abactinal side. Secondary tubercles are 

 scarcely found on the abactinal side, at most a small one near the primary tubercle 

 on 2 — 4 plates above the ambitus. — The development of the epistroma round the 

 primary interambulacral tubercles and in the ambulacra is rather variable, upon the 

 whole not very strong (PL XV Fig. 3). 



The genital openings appear early; they are present in the youngest specimen 

 in hand, 14 mm. diameter. They lie mostly nearer to the middle of the plate than 

 to the ape.x; so close to the apex as in the specimen figured in the xHassler» Echini, 

 PI. I. Fig. 4, I have never seen them. In some few specimens I have found a double 

 pore in one of the plates. — The ocular pore is generally not to be seen from 

 above, a knob growing out and covering it. This knob is connected with the outer 

 end of the plate across the pore, and a partition wall is formed which may con- 

 tinue down into the ocular pore, which is thus divided into two more or less distant 

 pores. The same peculiar structure — first described by LovÉN in Tetrapygus niger 

 {Echinocidaris nigra) (Études s. 1. Éch. p. 6"]) — is found in the other species of 

 Arbacia, more or less developed. In the A. crassispina, described below, this fea- 

 ture is especially prominent (PI. XV Figs. 9, 11). — Though generally the ocular 



Type-specimen of A. alternans (Troschel) 



