402 
THE VOYAGE OF H.M.S. CHALLENGER. 
Hexactinellids ; that they are not so proves simply that the conditions for their 
preservation were not so favourable as Schulze seems to assume ; both Lyssacina and 
Monaxonida are absent because they d'o not possess consistent skeletons. 
It is quite possible also that Monaxonida and Lyssacina are less uncommon than 
supposed in Jurassic rocks, the difficulty may be to find them ; like the skeletons of the 
Lithistida and Dictyonine Hexactinellids their spicules would probably be converted into 
carbonate of lime ; and while whole skeletons like those of the Lithistida and Dictyonina 
may be extracted from the limestone in which they are preserved, single spicules can be 
so obtained only very exceptionally. 
The relation between the nature of the sea-floor passed over by the Challenger and 
the number of species dredged is shown by the following list : — 
From — 
13 Stations 
on hard ground 
3 
species were 
obtained, 
or 23 per cent. 
9 
>5 
gravel 
3 
33 
33 
33-3 
33 
8 
JJ 
sand 
5 
33 
33 
62-5 
33 
4 
)5 
mud 
0 
33 
33 
20 
JJ 
coral mud 
7 
33 
33 
35 
33 
21 
3? 
green sand and mud 8 
33 
33 
38-1 
33 
30 
33 
volcanic mud 
6 
33 
33 
20 
33 
48 
33 
blue mud and clay 20 
33 
33 
41-6 
33 
55 
33 
Globigerina ooze 
3 
33 
53 
5-46 
33 
10 
33 
Pteropod ooze 
3 
33 
33 
30 
33 
4 
33 
Diatom ooze 
1 
33 
33 
25 
33 
4 
33 
Eadiolarian ooze 
0 
33 
3> 
50 
33 
red clay 
4 
33 
33 
8 
3? 
Thus mud and Eadiolarian ooze yielded no Tetractinellida ; the other deposits may be 
represented according to their richness (inversely, commencing with the poorest) in the 
following order : — 
Globigerina ooze, 
2 
Gravel, 
. 12 
Red clay, 
3 
Coral mud. 
. 13 
Volcanic mud, . 
7 
Green sand, 
. 14 
Hard ground. 
8 
Blue mud, . 
. ' . .15 
Diatom ooze, 
9 
Sand, 
.23 
Pteropod ooze, . 
. 11 
The numbers attached to each deposit are the nearest whole numbers to multiples 
of 2, which is taken to represent the richness of Globigerina ooze. 
If instead of species the same comparison is made for stations, we obtain the following 
results ; — 
