390 Professor Edward Forbes on 



great depths. But this is not the case ; no continuity of mud, for 

 instance, enables Scrobicularia to live beyond its bounds, or the 

 characteristic Rissose of the gravelly parts of the Laminarian zone to 

 extend themselves into the deep sea. 



The conditions of the sea-bottom, which are most favourable to 

 variety of species, may best be illustrated by referring to those 

 dredging papers in which the number of species of either univalve 

 or bivalve testacea taken alive exceeded ten. In the southernmost 

 of the districts within the area under consideration, out of eighteen 

 papers, ten come under the category. Three of these belong to the 

 Laminarian zone ; five to the Coralline region, and two to the upper 

 region of deep sea-corals. The three first mentioned are all from a 

 muddy and stony, or gravelly bottom with weed, and within two 

 miles of the shore : their number of univalves exceeds that of bivalves ; 

 in all three, the number of living univalves is very high, being fifteen 

 and above ; and in two of them, the numbers of living bivalves are 

 respectively ten and nineteen, and of dead, nine and twelve. Of the 

 five papers from the Coralline zone, four are within three miles 

 from the shore ; three of these are from bottoms more or less stony 

 and gravelly, in one instance mingled with nullipore ; and one is 

 from a floor of shell-sand. They are also very prolific ; one in dead 

 and living univalves, one in dead and living bivalves, and two equally 

 so in bivalves and univalves. The fifth of these Coralline papers is 

 from a depth of 30 fathoms and under, and a bottom of sand and 

 gravel, at a distance of 1 1 miles from shore ; it exhibits a great pre- 

 ponderance of bivalves, and an equal number of species takeiv dead 

 and alive. The two deep-sea papers are from a depth of 50 

 fathoms, on a sandy bottom, 60 miles from land ; they scarcely come 

 under the head of prolific papers, since few living species were taken, 

 though many dead, the number of dead univalves predominating in 

 the one instance, and of dead bivalves in the other, respectively 

 seventeen and twenty, both high numbers. 



Twenty-six papers from the Irish sea relate to a sufficiently 

 limited range in depth, to admit of a similar inquiry. Of these, 

 eight included more than ten species of univalves and bivalves, or 

 both. Two are from the Laminarian zone, and within two miles 

 from shore ; in the one instance, where the bottom was gravelly 

 and stony, univalves prevail, and those alive ; in the other, where 

 it was sandy, bivalves prevail, and those mostly dead. The remain- 

 ing five papers are from the Coralline zone ; in three of them, where 

 the bottom was a scallop bank several miles from shore, the number 

 of both bivalves and univalves taken alive, was very inconsiderable, 

 reaching, in one instance, respectively to twenty-one and twenty- 

 seven. In one, from a nullipore bottom, one mile from shore, uni- 

 valves prevail, but bivalves are also abundant. In one, on a gravelly 

 and stony bottom near shore, bivalves prevail (the numbers being 

 twenty-four living and twenty-five dead), but univalves are also 

 plentiful. 



