REPORT ON THE TETRACTINELLIDA. 
401 
true then we must expect a change in the bathymetrical distribution of deep-sea forms 
as they are traced backwards in time ; a change which may be represented by a curve, 
with the depth for its ordinate and time for the abscissa, and traceable to an origin in 
the littoral zone. 
The geologist then must not expect the zoologist to furnish him with tables for 
determining the probable depths at which ancient sediments have been deposited ; these 
he must discover from other data, which are not wanting, and may then be able to furnish 
the zoologist with data for the construction of a bathymetrical curve changing with the 
time. 
Another fallacy common among geologists is to suppose that a comparative abundance 
of Hexactinellids and Tetractinellids compared with Monaxonids in any deposit is an 
indication of great depth ; this supposition is confuted even by the facts of existing 
bathymetrical distribution, which show that at all depths above 1000 fathoms the 
Monaxonids are the ascendant group ; the comparative rarity of this last group of Sponges 
in stratified deposits is to be explained on quite other grounds — (l) By the fact that they, 
like the Choristida, are not furnished with a coherent skeleton, so that all that remains 
after their decay is loose and scattered spicules, which, being in the majority of cases of 
small size, are readily dissolved, and thus totally disappear, while when they are com- 
paratively large they are liable to become mixed with precisely similar spicules of the 
Choristida, and thus cannot be separately identified. Another explanation, applicable 
however to only a few cases, is the erroneous identification occasionally made of some fossil 
Monaxonids with Calcareous sponges. 
The absence of Lyssacine Hexactinellids is to be explained in the same way as the 
absence of Monaxonid spicules. 
Schulze, after stating that the Hexactinellida of abyssal depths are almost exclusively 
Lyssacina, adds : — “ The conclusion therefore seems warranted that in ancient times also 
the Lyssacina predominantly occurred in the greater depths, while the more differentiated 
Dictyonina inhabited as they now do relatively shallower water at no great distance from 
the coasts. Now if one may assume that the deepest regions of the great oceans have 
remained permanently covered by water since the Palaeozoic period, while the shallower 
regions near the continents were here and there raised above water, we can understand 
why we find in certain Jurassic and Cretaceous deposits so many and highly differentiated 
Dictyonina, but very slight hints of Lyssacina, even under circumstances that would not 
preclude their preservation, or at least that of their characteristic spicules.”^ 
This hypothesis it appears to me cannot be sustained in face of the fact that 
Monaxonids are as conspicuously absent as Lyssacina from Jurassic deposits, while if 
Schulze’s view that the existing is a fair representation of the ancient distribution be 
true, then Monaxonids ought to be abundantly associated with the fossil Lithistids and 
1 Report on the Hexactinellida, Zool. Chall. Exp., part liii. p. 496. 
(ZOOL. CHALL. EXP. — PART LXIII. 1888.) 
Rrr 51 
