438 Mr. H. J. Carter on 



with more or less lanceolate or, rather, "fixed bayonet" end, 

 like that just described, sometimes serrated, with which tlie 

 cribriform sarcode of the dermis is charged. Taking the 

 phnce of the flesh-spicule in the non-calcareous sponges, and 

 thus also strengthening this structure, while it acts in combi- 

 nation with the sarcode as a kind of cement in binding down 

 the larger spicules of the surface generally, it has been termed 

 by Hackel " Stabchen-Mortei." Hence these spicules may 

 be termed " mortar-spicules," so that when this term is 

 used the reader will know what is meant by it. This form 

 again sometimes attains a larger size, when, as is the wont of 

 the dermal acerates generally, they become mingled with the 

 internal ends of the spicules of the mouth and thus form part 

 of the proximal end of the peristome. 



But the staple and, therefore, most characteristic spicule of 

 most calcareous sponges is a " radiate," which may be regular 

 or irregular ; that is to say, when the arms or rays are all 

 equal in size and all separated from each other at equal angles 

 it may be termed " regular," and when the reverse " irre- 

 gular." 



The radiates, again^ may be divided into three-armed and 

 four-armed spicules, i. e. triradiates and quadriradiates, which 

 may be more or less equally mixed together according to the 

 species or their position in that species. 



Noticing the triradiates first, it may be observed that they 

 are generally more or less elevated in the centre, so that if on 

 a level surface with the points of their arms downwards they 

 would rest on these points, a feature which attains its maxi- 

 mum in the peculiar form that characterizes the surface of 

 Clathrina tri'podifera^ as will be more particularly mentioned 

 hereafter. It may also be observed that when the triradiates 

 depart from their " regular " form they for tlie most part 

 become more or less bow-and-arrow shaped (whence they have 

 been termed "sagittal "), in which two of the arms may be 

 variously expanded laterally in a more or less curved or undu- 

 lating form, backwards or forwards, while the third remains 

 more or less straight, and hence will be termed the " shaft." 

 Under this form they may be generally small or gene- 

 rally large. Tlius, as before stated, when generally small 

 and numerous they form the skeletal structure of the radial 

 tube or chamber in the Sycones, which has been termed 

 " articulated ; " while when large, with long shafts, the latter 

 alone extending across the wall simulates that which has been 

 called " inarticulated." 



The " sagittal " form appears to be often used for binding- 

 down the subjacent spicular structure, and, following it up- 



