248 REPORT—1850. 
ormis: of these, Pecten danicus and Astarte elliptica are living inhabitants 
of the Scottish seas, the latter in places still abundant, the former very rarely 
taken alive, though the dead shells occur in such vast quantities, that we 
cannot but regard it as a species which has lived on since the glacial epoch, 
though gradually becoming reduced in numbers, and now very nearly ex- 
tinct. These shells often occur at considerable depths, and almost always 
on a bottom of dark pleistocene sand. Pecten islandicus is enumerated in’ 
Hebridian and Zetland dredging papers from depths of 30, 40, 50 and 90 
fathoms. That this remarkable species is extinct in our seas we can scarcely. 
doubt, but I have good reasons for surmising that its extinction has taken place 
at a period considerably later than that of several of its glacial companions. 
The colours of this Pecten, as well as of some other pleistocene fossils, are 
beautifully preserved, and the general aspect of the shells is very deceptive. 
Occasionally, fossils of older date, but in such a condition of petrifaction 
as can lead to no mistake respecting their origin, are brought up in the 
dredge. Thus Mr. MacAndrew has dredged the loose joints of Liassic pen- 
tacrinites off the Shiant Islands, and we have seen Oolitic testacea dredged in 
the sound between Scalpa and Raza. 
GENERAL CONSIDERATIONS. 
Numerical distribution of species in depth.—Of the species of Testaceous 
Mollusca enumerated in the preceding tables, I have assigned a range to 188 
in the Scottish, and 18% in the English sections. Of the 188 Scottish sub- 
littoral species, whose range in depth I venture to state, 96 are Gasteropo- 
dous Testacea, and 92 Acephala. Of these, 17 univalves and 11 bivalves 
inhabit the region between low-water mark and 15 fathoms, 7. 6. the Lami- 
narian zone; 8 univalves and 7 bivalves extend their range from within the 
Laminarian zone to a depth between 15 and 30 fathoms; 26 univalves and 
11 bivalves from the Laminarian zone to between 30 and 60 fathoms; and 
25 univalves and 53 bivalves, from the Laminariau zone to a depth between 
60 and 100 fathoms: 3 univalves and 4 bivalves are confined in their range 
between 15 and 30 fathoms, z. e. to the Coralline zone ; 1 univalve to between 
30 and 60 fathoms; 4 univalves and 1 bivalve to between 30 and 100 
fathoms; and 1 univalve and 1 bivalve to between 60 and 100 fathoms. 
Of the 183 in the English tables, 19 univalves and as many bivalves are 
from the Laminarian zone only; 45 univalves and 46 bivalves range from 
some point within the Laminarian zone to between 20 and 30 fathoms; 16 
univalves and 28 bivalves extend their range from the same region to between 
30 and 60 fathoms. 
It is evident that the capacity of bivalves to enjoy a great bathymetrical 
range exceeds considerably that of univalves. This power of enduring many 
conditions of depth, implies the power of adapting themselves to varying cir- 
cumstances, which cannot be supposed to exist without considerable varia- 
tion in the features of the individuals of such wide-ranging species. The 
rules which should guide us in determining the selection of diagnostic cha- 
racters from the shells of Acephalous mollusks, must consequently be less 
strict than those which should determine our selection of characters for the 
majority of Gasteropoda, and in the determination of fossil species this should 
constantly be borne in mind. The difference of power to range presented 
by univalves as compared with bivalves, has a further important bearing on 
paleontological inquiries, for it would indicate the probability of our not 
unfrequently finding geological formations connected together by the fossils 
of the one class of mollusea, whilst those of the other are altogether distinct, 
even in strata proximate in time. It is possible also, that by a careful de- 
