CORALLINE CRAO, 
89 
which are really characteristic Mediterranean shells, and are only 
marked as British in consequence of some rare occurrence/’ 
The Mollusca are such as flourish in the lower part of the 
larninarian and the upper part of the coralline zone. A small 
number of deep-water or littoral species are found, but individually 
they are not abundant, and they are also quite insufficient at any 
one spot to modify our impression of the general shoal-water 
character of the fauna. The average depth at which the living 
species of Coralline Crag Mollusca now fl.ourish, if we take only 
those of which the identification is not in dispute, is about 45 or 
50 fathoms, and we may safely calculate the depth at which this 
deposit was laid down to have been somewhere between 40 and 
60 fathoms. A greater depth would not have produced the 
abundant RissocB, a less depth would encourage a larger propor¬ 
tion of littoral and plant-eating species. 
Next in importance to the Mollusca come the Bryozoa, which 
occur in such profusion as seems to be quite unequalled in any 
other deposit in Britain, or probably in Europe. Busk described 
118 species, of which 76, or about 64 per cent., appear to be 
extinct. Among the extinct forms are the peculiar genera 
Alveolaria and Fascicularia (Fig. 3, p. 20), large massive bryozoa, 
characteristic of European Pliocene and Miocene deposits, and 
unlike anything now living. The living species amono* the 
Coralline Crag bryozoa have principally a southern distribution; 
a few are exclusively southern. But the species in this class have 
generally such a wide range, both ^horizontally and vertically, that 
they cannot be said to point to any very definite conclusions as to 
climatic conditions, or as to depth of water. The average depth 
from which the living Coralline Crag forms have been dredged 
appears to be about 65 fathoms, but a large proportion of them 
range from low-water mark to 160 fathoms. The essential con¬ 
ditions for bryozoa seem to be clear water and fairly stronf^ 
currents. Depth or temperature are apparently of less importance, 
and may vary considerably without much afifecting the character 
of the bryozoan fauna. 
True Corals are only represented by three species, all of which 
Fig. 6. 
Flahellum Woodii, E. & H. 
Natural size. 
Milne-Edwards considered to be extinct. Two of them, Spheno- 
trochus intermedins and Flahellum Woodii (Fig. 5), are solitary 
