CORBITELLA AND HETEROTELLA. 25 



Such hexactins, 200-300 /'- in axial length and 3-6 1>- in breadth 

 of rays near the center, were at places not uncommon. When 

 very small (under 100 !>- in axial length), as they were sometimes 

 found to be, they may deserve to be called smooth microxy- 

 hexactins. But these are quite rare. 



The dermalia are mostly sword-shaped hexactins (PL, figs. 

 17 and 18) of variable size and strength. While a small one 

 may measure only half a millimeter or thereabout in length, a 

 large one may be over one millimeter long. In thickness of rays 

 the variation ranges from 9 ,« up to 30 y-, as measured close to 

 the center. The comparatively short guard-rays and the pro- 

 longed blade-ray have rough ends, ultimately terminating in conical 

 points. The short hilt-ray, generally under 100,« in length, 

 tapers somewhat outwards and ends rounded ; it is either smooth 

 all over or sparingly beset with low microtubercles. Not un- 

 commonly the hilt-ray is reduced to a mere knob (PL, fig. 19) 

 or has even entirely disappeared ; the spicule is then a pentactin 

 with the unpaired ray much prolonged. It can nevertheless be 

 recognized as a dermalia on account of its arrangement in associa- 

 tion with other unmistakeable hexactin-dermalia. Moreover, dif- 

 ferent stages in the reduction of the hilt-ray are plentifully re- 

 presented in the varied length of that ray in different dermalia. 



Certain pentactins with much more slender rays, which I 

 have occasionally met with in isolated positions, gave me the 

 impression that I had before me elements of the gastral skeleton. 



The marginalia (PL, fig. 16) are hexactins in which the 

 distally directed ray is specially developed and the longest. They 

 again are quite variable in size and strength ; they are, I believe, 

 connected with the dermalia by all sorts of intermediate sizes and 

 forms. Tolerably well developed marginalia have the distal ray 



