MIDDLE CHALK—CONDITIONS OF DEPOSIT. 555 
Terebratula, Terebratulina and Crania . Of these the first two 
now range down to 2,000 fathoms, but are most abundant beyond 
500 fathoms. Of Terebratulince, T. caput-serpentis , which so 
closely resembles the Cretaceous species T. striata, ranges from 
0 to 1,180 fathoms, and T. Cailleti (which may be compared 
with T. lata ) ranges to nearly 500 fathoms. Crania is found 
in all depths between 0 and 1,500 fathoms. 
M. OEhlert remarks that of those which range beyond 500 metres 
(270 fathoms) the greater number are found between the depths 
of 500 and 1,500 metres (270 to 820 fathoms) ; these include 
six species of Terebratulina, six of Terebratula and Magellania, 
three of Megerlia, two of Rhynchonella, and one of Crania.* 
This would indicate an average depth of 540 fathoms for the 
Brachiopods of the Middle Chalk. 
Bryozoa. —Remains of these creatures are rarer in the Middle 
Chalk than in any other part of the formation. There are a few 
encrusting forms, and Homceosolen occurs sometimes in the 
zone of Terebratulina^ but other arborescent forms are very rarely 
found. At the present day these arborescent forms are decidedly 
most abundant in comparatively shallow water (50 to 150 fathoms). 
Some species, however, of both orders range down to 500 fathoms, 
and a few as far as 1,000. The rarity of Bryozoa in this 
part of the Chalk is consequently very suggestive of a greater 
depth than 500 fathoms. 
Crustacea. —These are very rare fossils in the Middle Chalk, 
and as at the present day the Decapods range down to great depths, 
some Brachyura to 800 fathoms, and some Macrura to 1,500 
fathoms and more, there must have been some special conditions 
that were unfavourable to Crustacean life. Possibly there was 
a deficiency of the right kind of food. 
Echinodermata. —- Echinoidea are fairly common fossils in 
the Middle Chalk, but only three of the genera survive to the 
present day—namely Cidaris, Salenia, and Hemiaster. Of these 
Cidaris (proper) does not range beyond 200 fathoms, but as 
Dorocidaris it extends to 870, Salenia ranges to 1,800 and 
Hemiaster to 800 fathoms. Of Micrasters there are two species 
in these beds, and the commonest is M. corbovis, a form which, 
as Dr. Gregory has pointed out to me, in its thin test and 
shallow-petalled ambulacra presents the aspect of a deeper sea 
species when contrasted with the forms in the lower zones of 
the TJpper Chalk. Calerites is closely allied to the modern deep 
* In Fischer’s Manuel de Conchyhologie, p. 1252. 
