14 BULLETIN 74, TJXITED STATES NATIONAL MUSEUM. 



In the relative proportions of the ])!uts of the test there seem to be no constant 

 (lifTeronccs, as is apparent from tiic moasuremcnts. In the c^eneral shape of tlie 

 test tiiere may perhaps prove to he a difi'erence. The hxrge specimen of the variety 

 is distinctly pentagonal in outline, the interambulacra being rather convex at the 

 ambitus, forming the (rounded) angles of the pentagon (Plate 10. fig. 2). In the 

 specimen of ;i6..5 mm. horizontal diameter tiiis feature is also indicated, though much 

 less distinct than in the large specimen. Whether this feature is observable also in 

 the two other specimens I can not state definitely, not having denuded them. 



The ambulacra are somewhat dill'erent, at least in the specimens before me. 

 In the original description of ahyssicola Agassizsays that "each plate carries a larger 

 exterior tul)ercle witli a smaller one nearer the abactinal edge, and sometimes a 

 third and fourth miliary between the two." This description agrees entirely with 

 the two specimens of the typical foi-m before me — excepting that the small inner 

 tubercle lies at the actinal edge (Plate 14, fig. 2). Considering, however, that, so 

 far as I know, this inner tubercle (when only one is found) is elsewhere in cidarids 

 never placed at the abactinal edge, but either in the middle of the plate or nearer 

 the actinal edge, I think it not unreasonable to suggest that "abactinal" here is a 

 la'psus calami for "actinal." In the variety I find the inner tubercle placed more 

 in the middle of the plate, and the miliary tubercles (carrying pedicellarite) along 

 the lower edge more numerous, forming, especially in the larger specimen, a com- 

 plete series along the lower edge. (Plate 14, figs. 3-4.) 



In the interambulacra I find the median space distinctly broader in the variety 

 than in the typical form. The deepening of the median line is distinct almost to the 

 peristome, that is to plates 1 and 2 in the typical form, scarcely visible below the 

 ambitus in the variety, that is to plates 3 and 4. 



The apical system does not seem to afl'ord any constant differences. In the 

 two specimens of 36 mm. horizontal diameter of the variety the anal plates send 

 out a prolongation reaching the ocular plates, which are thus in contact with the 

 periproct ; in the two other specimens this is not the case. Also the shape of the 

 ocular plates is somewhat variable; they seem upon the whole to be slightly larger 

 and less prominent than in the typical form. (Plate 15, figs. 3, 6, 9.) 



The peristome shows no distinguishing features. 



The radioles afford the most conspicuous difl'erences. In the typical form (Plate 

 7, figs 1-2; Plate S) they are somewhat fusiform, and attain their greatest diameter 

 "at about one-fifth the length of the spine from the base," as described by Agassiz 

 (p. 254). In the variety (Plates 9-10) they are quite cylindrical and generally 

 more slender. (In tlie large specimen they are, however, as stout as those of the 

 typical form.) The actinal radioles (Plate 15, figs. 4-5, 10-11) are a little more 

 widened in the variety than in the typical form, but this difference is not quite 

 constant; also in the typical form they may be just as much widened (Plate 15, 

 fig. 10). They are slightly curved and flattened on the proximal side. The edges 

 are generallj' distinctly seirate, though scarcely so much as in the one figured in 

 Plate 15, fig. 10. In transverse sections (Plate 16, fig. 11) the radioles are seen to 

 be finely sjjinous; though tlie.y ap})ear very smooth, they thus really differ conspicu- 

 ously from those of Calocidaris micans, in which the ostracum is quite smooth." 



"See Ingolf Echinoidca., pt. 1, pi. 11, fig. 24. 



