86 l'AXA.MIC DEEP SEA ECHINI. 



or among those of Hygrosoma available for comparison showing reasons 

 for the adoption of Hygrosoma as contrasted to Phormosoma. 



It is with a sigh of relief that Dr. Mortensen 1 comes to Pit. asterias, "the 

 last of the Echinothurids described from the 'Challenger'"; to be exact I 

 beg to call his attention to the fact that it is not the last species described. 

 ( If course he finds the description useless. Fortunately some of the 

 figures meet his approval, and a new genus is established for the species 

 based on characters which to me appear most trivial. 



Phokmosoma Wyv. Thorn. 

 Phormosoma hispidum, A A.g. 



Phormosoma hispidum. A. A.g., Bull. M. C. Z. 1898, XXXII, No. 5, p. 77. Plates VI, VII. 

 Plates A; B, fig. /; 30-42; 43, figs. 5, 6; 44-47; 48. figs. S-6; 49. 



This species is the Pacific representative of the Caribbean and Northern 

 Atlantic Ph. uranus. It has like it an extensive geographical distribution 

 from the Panamic to the Smith Californian district, but in comparatively 

 deeper water. Its range is from about 1000 to over 1300 fathoms, while 

 the Atlantic species has been found at a much less depth (399 fathoms). 

 An excellent series of specimens of this species was collected, varying from 

 15 mm. to 203 mm. in diameter (Pis. 30-38), the greater number being from 

 120 to 130 mm. in diameter (Pis. 31, fig. 3; 32; 33, fig. 3 ; 34, fig. 1- 

 35; 3G). In the largest specimen, 203 mm. in diameter (Pis. 37; 38) the 

 actinal system was 44 mm. in diameter (PI. 41, fig. .') and the apical 44 mm. 

 (PI. 39, fig. 3). The test is marked by the great width of the ambulacra] 

 s^ystem, which at the ambitus is no less than 57 mm., — nearly of the same 

 width as the interainbulacral area at the corresponding edge. 



It will be simpler to describe first the youngest specimens collected, and 

 trace the changes taking place with increasing size, until we reach the 

 largest specimens dredged by us. 



The .young stages of Phormosoma are less globular, more flattened, like 

 Diadema, than the older stages (PI. 30). The youngest specimen collected 

 (PI. 30, figs. 1, J) measured 15 mm. in diameter, Fig. 133 ; the actinal 

 system 5 mm. in diameter (Pis. 43, fig. 5; 44, fig. 1 ; 48, fig. ..'). with three 



1 " lugolf " Echinoidea, p. GO. 



