PARKER: METRIDIUM MARGINATUM. 263 



DiGLYPHic Type (two siphonoglyphs and two pairs of directives). 



In this type (Fig. 1) the two pairs of directives of course divide the 

 Dou-directives into two groups. I regret that, before it occurred to me 

 to determine the number of mesenteries in each of these two groups for 

 the 53 specimens of this type, some of the specimens were so far dis- 

 sected as to render them no longer serviceable for this enumeration. I 

 can therefore make a statement concerning this division in the cases of 

 only twenty specimens. 



In comparing these results, it will be observed, first, that the great 

 majority of individuals (40 in 53) possess four pairs of non-directives, 

 and, next, that the arrangement of these non-directives in ten cases out 

 of twenty is in two groups of two pairs each. This symmetrical 

 arrangement of the four pairs of non-directives in the diglyphic type 

 reproduces the assumed typical Hexactinian arrangement, and, since 

 the representatives of the other variations are comparatively so few in 

 numbers, this may be taken to be the only characteristic condition of 

 the diglyphic type. 



In the monoglyphic type (77 specimens), in addition to the one pair 

 of directives, there were from three to fourteen pairs of non-directives. 

 The frequency of their occurrence is shown in the following table : — 



Admitting the monoglyphic type to be derived from the diglyphic by 

 the conversion of a pair of directives into a pair of non-directives, 



