168 THE VOYAGE OF H.M.S. CHALLENGER 



Cho7-ology of the Genus Thoracaster. 

 a. Geographical distribution ; — 



Atlantic : One species between the parallels of 20° and 30° N. 



TJioracaster cylindratus off the west coast of Africa, between the 

 Canary Islands and the Cape Verde Islands. 



ft. Bathymetrical range : 2400 fathoms. 



y. Nature of the Sea-bottom : Globigerina ooze. 



CJwrological Synopsis of the Species. 



1. Thoracaster cylindratus, Sladen (PI. XXIX. figs. 1-6). 



TJioracaster cylindratus, Sladen, 1883, Journ. Linn. Soc. Lond. (Zool.), vol. xvii. p. 245. 



Rays five. R = G2nim. ; r = 21rum. E = 3r. 



Marginal contour stellate, with large disk and very narrow cylindrical rays, which 

 taper to a point. The disk is slightly inflated, forming a convex surface of low curvature. 

 The minor radius is in the proportion of 33-8 per cent. The interbrachial arcs are wide 

 and well rounded. 



The abactinal area is covered with small, compact, and closely crowded paxillae, which 

 are, however, confined entirely to the disk in consequence of the junction of the supero- 

 marginal plates in the median line along the whole of the free portion of the ray. The 

 paxillae are very small, and composed of six to ten small spinelets closely appressed into a 

 fascicule — the whole area appearing to the unaided eye almost like a uniformly granular 

 surface. The paxillae are a shade smaller in the centre of the disk, which they further 

 define by their arrangement, although no prominent epiproctal peak is produced. The 

 actinal area of the disk is slightly convex and slopes downwards at a small angle to the 

 prominent mouth-plates. 



The marginal plates are high, forming a gently rounded margin to the disk, the plates 

 curving slightly inward towards the abactinal and the actinal areas respectively. When 

 viewed from above they are seen to encroach on the abactinal area to a very slight degree, 

 and still less on the actinal. Along the free portion of the ray the supero-marginal plates 

 of the two sides of the ray meet in the median radial line and entirely encase the ray ; 



