PALAEONTOLOGY.
37
Professor Forbes gives the following Table to illustrate another important consi-deration in respect to the distribution of species; namely, its variation as regards thedifferent families of Testacea.
Distribution of British Forms in the several Zones of the JEtjean Sea.
FAMILIES.
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
Multivalves.
I
0
1
1
1
1
0
Patelliform univalves.
0
1
2
2
2
2
2
0
Tubular univalves.
1
1
0
0
0
0
0
0
Holostomatous spiral univalves . .
12
9
13
16
14
11
8
4
Siphonostomatous spiral univalves.
4
5
7
8
9
6
5
2
Testaceous Pteropoda and Nucleo-
l
brancliiata.
/ '
Brachiopoda.
0
0
0
0
0
Conchifera and Lamellibranchiata .
16
25
28
39
33
19
11
7
Total . . .
34
41
50
66
59
39
27
13
Per-centage
23
32
40
41
42
33
31
20
From this Table it appears that the least per-centage of the whole number ofTestacea is found in the highest and lowest zones,—the greatest in the 3rd, 4th, 5th,and a medium per-centage in the 2nd, 6th, and 7th.
The cause of these results may be traced to the small expansion or range of somespecies, as may be observed on comparing the number of British forms in the pre-ceding Table with the total numbers in the following.
Distribution of JEqean Shells in Depth.
FAMILIES.
Total.
II.
III.
IV.
V.
VI.
VII.
VIII.
Multivalves.
7
3
2
0
2
2
1
1
0
Patelliform univalves.
20
11
3
2
3
5
6
6
i
Tubular univalves.
6
4
4
2
2
1
1
2
2
Holostomatous spiral univalves . .
115
50
40
40
44
35
28
17
15
Siphonostomatous do. do. . . .
104
40
27
30
41
36
30
16
5
Testaceous Pteropoda and Nucleo-branchiata.
}l2
1
0
0
0
0
0
3
12
Brachiopoda.
8
0
0
0
2
4
5
7
3
Conchifera and Lamellibranchiata .
135
38
53
52
68
58
48
34
28
407
147
129
126
162
141
119
86
66
And it is evident, therefore, that the Geologist, in his reasonings on the fossil fauna;of past epochs, must take into consideration not only the total number of species, butalso the number of species in each family, in order to arrive at safe conclusions as toclimatal changes; and in doing so he must also bear in mind the great influence ofthe sea bottom in determining the presence of certain families of Testacea.
It will be readily believed that these zones of depth must be the scene of incessantchange, as in one place the mineral matter is washed away, and in another it isdeposited, increasing or decreasing the depth; and, in like manner, the same effectsmay be produced by the depression or elevation of the sea bottom from movementsof disturbance, movements which are still continuing, and which were so powerfullyexhibited in ancient epochs. In this manner the lateral distribution of a species isoften arrested by a change of the sea bottom, and its total annihilation produced by
i