M. K. A. Zittel on Fossil Calcispongice. 369 
Cambridge ( Pkaretrospongia ) belongs not to the calcareous 
but to the siliceous sponges. The form and position of the 
spicules and the observation that in fossil sponges originally 
siliceous skeletons often appear converted into calcite, decided 
Sollas to place Pkaretrospongia in the group Holorhaphidota. 
Carter* summarizes the arguments which render it impro¬ 
bable that Pkaretrospongia belongs to the Calcispongiae as 
follows:—1. All existing Calcispongiae are not only very 
small, but for the most part absolutely minute. 2. With the 
exception of half a dozen species, the skeleton of the Calci¬ 
spongiae consists of triradiates or quadriradiates, and the bacil¬ 
lar spicules are always straight, never curved in the form of 
an arc. 3. The Calcispongiae are so perishable, that immedi¬ 
ately after death they become diffluent, and indeed, in conse¬ 
quence of the absence of horny fibres and siliceous consti¬ 
tuents. 4. The spicules break up very rapidly even in 
Canada-balsam preparations, pass into watery spherules, and 
in a short time leave no trace of their existence. 
All these objections, raised with respect to Pkaretrospongia , 
apply also to the other fibrous sponges ; if they hold good, the 
possibility of the occurrence of any fossil Calcispongiae is 
most seriously shaken. 
In the first place, with regard to comparative size, it must 
be admitted that the living Calcispongiae rarely attain any con¬ 
siderable magnitude ; but nevertheless Hackel describes stocks 
from 50 to 100 millims. in height in all the three existing 
families ; and among the Leucones there are even individuals 
30-40 millims. long and 15-20 millims. broad. These 
latter, in size and external form, are almost undistinguishable 
from the most common forms of fossil fibrous sponges, such 
as Peronella and Corynella. Just as the recent Calcispongiae 
yield considerably in size to the other sponges, so also the 
fossil fibrous sponges are generally distinguished, in compari¬ 
son with the Hexactinellidae and Lithistidae, by their small 
dimensions. The genus Pkaretrospongia , described by Sollas, 
chances, with Pachytilodia , to contain the largest known 
forms of fibrous sponges. The difference of size between the 
living and fossil Calcispongiae is by no means so considerable 
as to exclude the possibility of their relationship. 
The objection relating to the form of the skeletal elements 
is of more weight. That in living Calcispongiae bacillar 
spicules only exceptionally (in eight species) form the skele¬ 
ton, cannot be disputed; but among the Ascones, as well as 
the Sycones and Leucones, there are genera consisting 
* Ann. & Mag. Nat. Hist. ser. 5, vol. i. (1878) p. 136. 
Ann. & Mag. N. Hist. Ser. 5. Vol. iii. 26 
