REPORT ON THE ECHINOIDEA . 101 



test, more or less wide according to the size of the tubercles. The structure of the 

 primary tubercles of the actinal surface, and their irregular arrangement in horizontal 

 and vertical rows is very characteristic of the genus Phormosoma, and resembles in 

 the most striking manner the arrangement and structure of the primary tubercles of 

 Archceocidaris among the Paleeechinidse, showing that Archceocidaris, and perhaps 

 Eocidaris and Pholidocidaris, are more closely related to the recent Echinothuridse 

 than the group of Pala3echinid8e, to which Melonites and the like belong, which, as 

 far as we can judge, from what we know of the structure of the apical system, and" 

 of the actinal membrane are more closely related to the Cidaridse. 



The presence of sheathed spines in two species of Phormosoma shows that this 

 character, which at first sight seems to separate so strikingly from the rest of the 

 species of the group Asthenosoma gruhii, is evidently one of little value, and which 

 may be more or less developed in specimens of the same species in the same state 

 of growth. 



In a specimen of Phormosomxi lucidentum, measuring nearly 160 mm. in diameter, the 

 abactinal system measured 22 mm., the actinal opening 42 mm.; while in a specimen of 

 Pliormosoma hursarium, measuring 100 mm. in diameter, the abactinal system measured 

 28 mm., the actinal opening 40 mm. The smaller specimens of these two species collected 

 from the same localities could readily be assigned to their respective species, yet more 

 abundant material may prove that the differences noticed, although important, are 

 simply individual characteristics partly due to age. 



Station 200. October 23, 1874. Lat. 6° 48' N., long. 122° 25' E. ; 255 fathoms; 

 mud. 



Station 205. November 13, 1874. Lat. 16° 42' N., long. 119° 22' E. ; 1050 

 fathoms; bottom temperature, 2 '4° C. ; grey ooze. 



Station 232. May 12, 1875. Lat. 35° 11' N., long. 139° 28' E. ; 345 fathoms; 

 bottom temperature,- 5 "0° C, ; sandy mud. 



'"^ Pliormosoma hoplacantlia (Pis. XL, XIL, XIL'' figs. 10-13; PL XXXIX. figs. 

 3-7 ; PI. XL. figs. 37, 38 ; PI. XLIII. fig. 1 ; PL XLIV. figs. 28-31). 



Phormosoma hoplacantlia, Wy. Thomson, 1877, Voyage of Cball., Atlantic, vol. i. p. 148, 

 fig. 35. 



This is the largest Sea-urchin with which I am acquainted, it measures no less than 

 312 mm. in diameter, and when fully expanded, must have been a striking object. This 

 species is remarkable for the large size of the primary tubercles, arranged both on the 

 actinal and abactinal surface of the interambulacral areas (PL XIL" figs. 11-13) in 

 horizontal rows ; on the abactinal surface they are distant, separated by large secondaries 

 and miliaries, irregularly arranged on the coronal plates (PL XIL'' fig. 13). Towards 



