HALICHONDRIA. 101 



is that of half of a short, stout, pear-shaped mass. 

 The surface is even and smooth, and the oscula are 

 small and rather few in number. The dermal struc- 

 tures afford the most efficient specific characters for 

 the discrimination of the species. The fasciculi of the 

 tension spicula are extremely abundant ; they are com- 

 posed of numerous spicula closely packed in parallel 

 lines, forming wide, flat groups, crossing each other in 

 every possible direction. The spicula of which they 

 are composed are equable in size, but variable to some 

 extent in form ; those in the fullest state of develop- 

 ment have decidedly hastate terminations, while others 

 present no terminal dilatation, but terminate suddenly 

 and acutely instead of hemispherically. The tridentate, 

 equi-anchorate, retentive spicula, although few in 

 number, are very characteristic ; they are small, not 

 exceeding - 7 --Q- inch in length, and require a power of 

 about 400 linear to render them distinct to the eye ; 

 the teeth of the spiculum are comparatively large and 

 long, each pair nearly the length of half that of the 

 shaft, so that their apices nearly meet each other, and 

 they are so widely expanded that each end nearly 

 represents a semicircle. This description more espe- 

 cially applies to the greatest number of these spicula, 

 the shortest of them. The longest have a greater 

 space intervening between the teeth of the two ends 

 of the spiculum. One of the longest measured y^j 

 inch in length, and one of the shortest sort measured 

 g-yy inch in length. The bihamate, retentive spicula are 

 not so numerous as the bidentate, equi-anchorate ones ; 

 they are small and of very slender structure, and are 

 not especially characteristic. One of the largest 

 measured -Q^Q inch in length, and one of the smaller, 



