502 E. A. MINCHIN. 



affinities as represented graphically in my former paper in the 

 form of a genealogical tree. The first family would be the 

 Clathrinidse, and includes the genera Clathrina and 

 Ascandra. The second familv would be the Leucoso- 

 leniidsBj to include Leucosolenia and probably Ascyssa. 

 Their systematic position is as follows : 



Class CALCAREA. 



Sub-class H0M0C(ELA. 



Order Ascones. 



I. Family Clathrinidse. 



Genera Clathriua and Ascandra. 



II. Family Leucosoleniidse. 



Genera Leucosolenia and (?) Ascyssa. 



II. Descriptive : Observations upon the Development of 



THE Spicules. 



Preliminary Remarks. — For the sake of clearness it may 

 be permitted to anticipate in one point the results of the in- 

 vestigations of which the description follows, with referencCj 

 namely, to the relations between the triradiate and quadri- 

 radiate spicules. As is well known, each triradiate spicule is 

 formed of a system of three rays, and each ray is placed tan- 

 gentially in the body-wall, entirely enveloped in the dermal 

 layer, and never projecting freely from it, neither towards the 

 exterior nor the interior. Typically one ray — the posterior 

 ray — points away from the osculum, and therefore downwards, 

 in an erect olynthus ; while the other two, the lateral rays, slant 

 upwards. In the species we are about to consider the three 

 rays meet at equal angles, and the posterior ray is only marked 

 out from the lateral rays by its position in the sponge, and 

 sometimes also by its greater length (CI. blanca). The three 

 rays do not, however, lie all exactly in the same plane; but, as 

 might be expected from their position in the wall of a cylinder, 

 a plane containing any two of the rays is met by the third at 



