Boone, Echinodermata, Cruises of "Ara" and "Alva" 157 



somite. These spines sometimes occur in the sequence 4, 4, 3, 4, 3, 

 or 4, 3, 3, 4, 3, 3, 4, and sometimes 4, 3, 4, 3, the fourth or dorsal 

 spine being definitely longer where it is alternated by two three- 

 spined somites than it is when alternated by only one three-spined 

 somite. The arm spines are of two distinct types: (a) the com- 

 pressed, claviform, which occur uniformly in the ventral and next 

 to ventral spines, and (b) the tapered, cylindrical forms which 

 occur in the third or subdorsal and dorsal spines. The claviform 



Text figure 8. — Ophioconm ivendtii Muller and Troschel, arm-spines X 5. 



spines are proximally subcylindrical, but abruptly become de- 

 cidedly compressed in the proximal-distal axis of the arm plate, 

 the broad surfaces of the spine having also this direction; the 

 tips of these spines are truncate, laminate, in the young, thickish 

 and sometimes slightly expanded fanwise in the large specimen. 

 The cylindrical spines are thick throughout their length, although 

 tapering distally, with truncate or bluntly rounded apices. In the 

 very young specimen these spines are more swollen medially than 

 in the older specimen and their apices are more convex. The 

 dorsal arm plates are wider distally than long, fan-shaped, with 

 the proximal margin short, the paired lateral margins obliquely 

 divergent, thence to the widely convex distal margins. 



The young specimen from the same locality has a disk diameter 

 of 5 millimeters, and all five arms incomplete, but with the longest 

 broken arm showing fifteen somites, the proximal five of which 

 are within the radius of the disk, each of which somites bear three 

 spines, two claviform and one cylindrical ; the eleven somites be- 

 yond the disk each possess in regular alternation 4, 3, 4, 3 arm 

 spines. Even in so young a specimen these spines show distinctly 



