EEPORT ON THE ECHINOIDEA. 141 



extent even in Pourtalesia laguncula, the posterior interambulacral zone on the abactinal 

 surface between the apical and the anal system is composed of nearly hexagonal plates, 

 increasing very gradually in size from the abactinal system to the anal system. This 

 uniformity of the plates is, however, in Pourtalesia entirely limited to the odd posterior 

 interambulacrum, while in Spatagocystis the plates of the other zones become more 

 uniform in size. This structural feature is also connected with the decrease in size 

 of the actinal groove, as seen from Spatagocystis, Echinocrepis, Urechinus, and 

 Cystechinus. The plates of the actinal plastron between the actinostome and the 

 anal snout are also far less elongate than in Pourtalesia proper, though these plates 

 are evidently the first to become diflferentiated, and appear to be the first trace of 

 a passage between the Galeritidse and the Spatangidse proper, as far as we can trace 

 this from the fossil genera. It is interesting in the recent Pourtalesise to compare 

 the actinal plastrons of such genera as Pourtalesia, Echinocrepis, Spatagocystis, and 

 Cystechinus, in connection with that of the Dysasteridse, Galeritidse, Hemiasteridaj, 

 and Spatangina proper. Subanal fasciole indistinct. 



'^Spatagocystis ehallengeri (Pis. XXVI. ; XXVI.'' ; XXXIX. fig. 37; PI. XLI. 

 fig. 40; PL XLII. figs. 10-12 ; PI. XLV. figs. 37-43). 



Spatac/ocystis Challengeri, A. Agassiz, 1879, Proc. Am. Acad,, vol. xiv. p. 206. 



The outline of the test seen from above (PI. XXVI. figs. 3, 9) is regularly ovoid, slightly 

 indented anteriorly, in profile arched, faUing towards the posterior extremity (PI. XXVI. 

 fig. 1), and quite abruptly truncated anteriorly. The actinal floor is flattened anteriorly, 

 with a prominent keel extending from the actinostome to the extremity of the anal snout 

 (PI. XXVI. figs. 1,2), the ambitus gracefully arched. The test is thin, quite brittle. 



AU the specimens collected as well as the fragments of tests were of a pinkish-violet 

 tint. The genital organs consist of large yellowish clusters hanging far down from' the 

 abactinal region by long slender ducts, the diflerent clusters of the genital organs quite 

 distinct. The lateral interambulacra are very broad (PI. XXVI.'' fig. 9), the coronal 

 plates are broad, and carry primary tubercles (PL XXVI. " figs. 2, 4, 12, 15, 16) uniformly 

 scattered over them, carrying fine slender spines (PL XXVI. figs. 1-5). The whole test 

 closely covered with miliary and secondary spines. 



On the actinal surface the spines are somewhat larger (PL XXVI. fig. 2) ; the 

 actinostome is small and deeply sunken (PL XXVI. fig. 2 ; PL XXVI." fig. 2). The 

 edges of the deeply-sunken actinal groove are quite well defined on the actinal side, 

 the groove resembles the elliptical anal groove of Echinohrissus. Like the anal groove 

 of some species of that genus it does not run indistinctly into a broad groove, and is 

 limited to one side of the test. 



In Pourtalesia proper the actinal groove passes very gradually into a broad anterior 

 groove, which occupies the greater part of the anterior extremity of the test, while in 



