388 Professor Edward Forbes on 



oblonga^ and, very lately, Ledu thraciaformis ; of these Pecten Dani- 

 cus 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 

 extinct. These shells often occur at considerable depths, and almost 

 always on a bottom of dark pleistocene sand. Pecten islandicus is 

 enumerated in Hebridean 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 con- 

 siderably 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 petri- 

 faction as can lead to no mistake respecting their origin, are brought 

 up in the dredge. 'Thus Mr Mac Andrew has dredged the loose 

 joints of Liassic pentacrinites, 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 

 Testacea MoUusca enumerated in the preceding tables, I have 

 assigned a range to 188 in the Scottish, and 183 in the English 

 sections. Of the 188 Scottish sublittoral species, whose. range in 

 depth I venture to state, ninety-six are Gasteropodous testacea, and 

 ninety-two Acephala. Of these seventeen univalves and eleven 

 bivalves inhabit the region between low water mark and 15 fathoms, 

 i.e., the Laminar ian zone ; eight univalves and seven bivalves extend 

 their range from within the Laminarian zone to a depth between 15 

 and 30 fathoms ; twenty-six univalves and eleven bivalves from the 

 Laminarian zone to between 30 and 60 fathoms ; and twenty-five 

 univalves and fifty-three bivalves from the Laminarian zone to a 

 depth between 60 and 100 fathoms ; three univalves and four bivalves 

 are confined in their range between 15 and 30 fathoms, i.e., to the 

 Coralline zone ; one univalve to between 30 and 60 fathoms ; four 

 univalves and one bivalve to between 30 and 100 fathoms ; and one 

 univalve and one bivalve to between 60 and 100 fathoms. 



Of the 183 in the English tables, nineteen univalves and as 

 many bivalves are from the Laminarian zone only ; forty-five uni- 

 valves and forty-six bivalves range from some point within the 

 Laminarian zone to between 20 and 30 fathoms ; sixteen univalves 

 and twenty-eight bivalves extend their range from the same region to 

 between 30 and 60 fathoms. 



