PARKER: METRIDIUM MARGINATUM. 263 
Diernvrnio Tyre (two siphonoglyphs and two pairs of directives). 
Pairs of Non-directives . . . . 4 5 6 7 8 9 10 
Number of Cases observed . . 40 4 9 3 2 0 1 
In this type (Fig. 1) the two pairs of directives of course divide the 
non-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. 
Groups of Non-directives. .|1+7/2+2/2+3/2+4 27 5 27 6 44-6 
Number of Cases observed . 1 10 2 2 3 1 1 
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-direotives 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 diglyphie type. 
In the monoglyphie 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: — 
Mowoarvyruro Typer (one siphonoglyph and one pair of directives). 
Pairs of Non-directives . . 3 465 6 7 | 8 | 9 | 10} 11) 12] 18 | 14 
| 
| 
| 
Number of Cases observed .| 1 | 4 a 19 21) 5 JE 1:590 106€ T 
Admitting the monoglyphie type to be derived from the diglyphie by 
the conversion of a pair of directives into a pair of non-directives, 
