52 DE. G. J. HINDE ON CALCISPONGES [Feb. I9OO, 



peculiar spicules described below. Owing to its delicate character 

 the dermal layer can be seldom shown in microscopic sections, but 

 fragments of it may be readily flaked off with a needle and mounted 

 for examination. It appears to consist mainly of elongate, smooth, 

 nearly cylindrical rod-like spicules, pointed, either lance-shaped or 

 styliform, at both ends (PI. Ill, figs. 43 & 44). On the exterior surface 

 these spicules are disposed generally parallel in the direction of the 

 length of the sponge, thus forming a sort of surface-thatch (PI. IV, 

 fig. 1). The spicules are not attached together in any way, and they 

 may be readily separated by treatment in water with a camel's-hair 

 brush, but they are so fragile that it is rare to obtain any entire ; 

 the longest fragment measured 061 mm. by 0-01 mm. in thickness. 



The inner surface of the dermal layer, in contact with the main 

 skeleton, is very uneven, with irregular ridges and intermediate 

 furrows, and it is penetrated at irregular intervals by small holes 

 with circular apertures from 0-05 to O075 mm. wide. At first 

 these holes were supposed to be connected with inhalant pores, but 

 in reality they appear to be due to the free apical rays of the mesh- 

 spicules which had penetrated into the dermal layer, and had left 

 their moulds when this was removed from the sponge. 



The inner portion of the dermal layer also consists of cylindrical 

 spicules, with pointed ends, smaller than those of the exterior, and, 

 unlike these latter, loosely intermingled without arrangement. 



In addition to the simple rod-like spicules, the inner portion of 

 the dermal layer contains a subordinate number of minute three- 

 and four-rayed spicules of various forms,' which appear to be 

 common also to the basal layer. The simplest of these are three- 

 rayed spicules of types common in recent calcisponges generally 

 (PL III, figs. 4-15). The spicular rays are usually straight and 

 smooth, tapering very gradually. Some are regular, having the rays 

 of equal length ; others are sagittate : while in others all the rays 

 are unequal. Rarely the rays are curved (PI. Ill, fig. 15), with an 

 occasional spine. The rays of these spicules vary in length between 

 0-004 and 0-225 mm. 



The peculiar three-rayed spicules, known as ' tuning-fork ' spicules, 

 in which the paired rays are parallel to each other and the third ray 

 extends in the opposite direction, forming the shaft of the fork, are 

 present, but of somewhat rare occurrence (PI. Ill, figs. 30-38). 

 The paired rays are relatively short and terminate obtusely, and the 

 shaft-ray is nearly cylindrical. These spicules are frequently eroded 

 and fretted, in marked contrast to the smooth glistening surfaces 

 of other spicules, equally as minute and delicate, with which they 

 are associated. The forked rays of these spicules range between 

 0-016 and 0-04 mm. in length, and are about 0-004 mm. in width, 

 while the shafts are from 0*02 to 0'17 mm. long, but it is doubtful 

 whether any are entire. 



This type of spicule * was first recognized in a recent calcisponge 

 from the Australian seas ; it has since been found in fossil Pharetron 



1 Bowerbank, ' Monogr. of Brit. Spong.' Kay Soc. vol. i (1864) p. 268 &pl. x, 

 fig. 237. 



