REPORT ON THE OPHIUROIDEA. 213 



shape, with a rounded angle inward, joined their whole length, except their inner ends, 

 which are separated by a wedge scale ; interl)rachial spaces below with a marginal 

 constriction, and usually covered by four large rounded scales ; there are a few short 

 smooth disk spines. Seven slender, translucent, sharp, not thorny arm spines ; lengths 

 to that of an arm joint, 1'3, 1'3, 1, 1," G, "4, '4 : "5. Beyond the basal joints there are but 

 six spines, whereof the u})per are shorter than those described above. One very small, 

 narrow, pointed tentacle scale. Colour in alcohol, nearly white. 



Ojihiothcmvnus vicariiis bears a general resemblance to this species, but is distinguished 

 by narrower side mouth shields and by long and numerous disk spines. 



Station 142, Agulhas Bank.— December 18, 1873; lat. 35° 4' S., long. 18° 37' E. ; 

 150 fathoms; sand. 



Species of Ophiotliamnus not herein described. 



Ophiothamnus affinis, Ljn., Dr. Goes, Oph. Of Kong. Akad., p. 622, 1871. 

 Portugal ; 790 fathoms. 



Opliioihrix. 



Opliiothrix, Miill. & Tr., Weig. Archiv, vol. vi., 1840. 



Disk set with thorny grains, very short, spines crowned with thorns, or spines with 

 thorns at the sides and top. Eadial shields like large, triangular swellings, each bounded 

 on its two inner sides l^y ridges in the skin of the back. Numerous crowded tooth papillse 

 forming a vertical oval. Teeth. No mouth papillse. Spines numerous (five to ten) (often 

 three times as long as the joints), flattened, more or less glassy, thorny, having a central 

 shaft with slender side-spurs from it. A small, spine-like tentacle scale. The base of 

 the jaw pierced with a hole, from a want of perfect union between the two pieces of the 

 mouth frames. Interbrachial spaces swelled out like lobes. Two genital openings 

 beginning outside the mouth shields. Outer arm joints with hooks. 



We have here a type separated by a gap from genera previously described. Contrasted 

 with the rather loose and feeble scaling are the very large, three-sided radial shields with 

 projecting knobs at their outer ends, where they are articulated with the clubbed, knobby 

 heads of the long, stout, rounded, and slightly curved genital plates (PL XLII. fig. 5, o). 

 To this last is attached a great, almost semicircular genital scale (;;), which is 

 continued to the mouth shield («) by an additional scale. The heads of the genital 

 plates nearly meet over the top of the arm, which is composed of peculiar arm bones. 

 Beginning at the third free bone, each has an upper forward projection or apophysis fitting 

 into a slot in the upper hinder end of the next bone. Thus the joints are interlocked in 

 a way that may give a fulcrum for the powerful muscular action called for in the rapid 

 whip-like motion of the arm of Opkiothrix. By this peculiar locking contrivance, the 



(ZOOL. CHALL. EXP. — PART SIV. — 1882.) 28 



