GEODITES. 149 
are but poorly preserved. I have been unable to detect any of a vermiform figure 
as represented by Mr. Carter. 
This species differs from H. Armstrongi in its mode of growth, in the more 
robust character of the fibres, and in the somewhat greater thickness of the 
spicules. 
Distribution.—Carboniferous: Upper part of Lower Limestone series; Cun- 
ningham Baidland, Law Quarry, Dalry, Ayrshire. 
Sub-Order.—TEtRACTINELLIDA. 
Genus.—Geropitsrs, Carter. 
1871. Annals and Mag. Nat. Hist., ser. 4, vol. vii, p. 129. 
Generic Characters—Sponges consisting of skeletal-spicules with bifid or trifid 
summit-rays and elongated shafts, and also of large acerate spicules and a dermal 
layer of minute globate or reniform spicules. 
No formal definition of this genus was given by Mr. Carter, who proposed it 
to include detached spicules of the forms indicated, which are similar to those 
present in the existing genus, Geodia, Lam., and, with the exception of the 
dermal globular spicules, in Stelletta, Schmidt, and other allied genera. As the 
differences which characterise the recent genera of this group (of which Geodia 
may be accepted as the representative form) cannot be recognised in the detached 
fossil spicules, it seems preferable to adopt for them the common generic term 
proposed by Mr. Carter. 
The acerate spicules in different species of this genus vary but little in form, 
and no satisfactory distinctions can be based on their relative proportions ; but the 
characters of the bifid and trifid spicules appear to be sufficiently constant to 
permit specific distinctions to be based on them. 
Owing to the fragile manner in which the spicules of this group of Sponges 
are held together by perishable spongin, it is extremely rare to find them in their 
natural positions in the fossil state, and their occurrence can only be recognised 
from the detached spicules. These make their first appearance in the Lower 
Carboniferous of Ayrshire; they occur also in the Lias of the Tyrol, and are 
extremely abundant in the Cretaceous strata of Brita and Germany. 
