PALEONTOLOGY.

35

distribution of land animals, namely,—climate, mineral structure, and elevation,—these primary influences being modified by several secondary or local causes.

In the Eastern Mediterranean, eight well-marked regions of depth were eachcharacterized by its peculiar fauna, and wherever plants were present, by its flora.These regions are distinguished from each other hy the associations of the speciesthey severally include; which assemblages include some species which are only foundin that particular zone, some which existed in preceding but do not continue intosubsequent zones, and some which, beginning in that zone, continue to exist insucceeding zones.

In this manner certain species have their maximum of development in each zone,and being there most prolific in individuals, may be regarded as especially charac-teristic. The sea bottom, in its mineral character, has generally a certain uniformityin each zone, that uniformity being more decided in the lower than in the upperzones. The deeper zones are greatest in extent, so that whilst the first or mostsuperficial is but 12 feet, the eighth or lowest is above 700 feet in perpendicularrange; and hence the more purely littoral animals were more subject to limitationthan those of deep waters.

First Region , or Littoral Zone .—This is the least extensive, as two fathoms maybe considered its inferior limit, and its mineral nature is as various as the coast line ;but limited as it is, there are well-marked subdivisions. The variety of its bottomnecessarily produces, according as it is sand, rock, or mud, a variation in theassociation of its several species. That portion which, forming the water-mark, isleft exposed to the air during the ebb of the tide, presents necessarily species peculiarto itself; species, in fact, which can resist the effect of so great a change, by eitherclosing their valves or burying themselves in the mud or sand. In the lower portionof this zone are the most characteristic species of a locality, and the colours both ofshells and animals are most fully and beautifully developed. As local influences, suchas variation of sea bottom, differences in tides and currents, flowing-in of rivers, &c.,now induce a peculiar or local distribution of species, the same effects must havebeen produced from similar causes in the ancient faunae, and the occurrence thereforeof abrupt and frequent changes, both in the mineral and zoological charactersof a formation, will naturally lead to the detection of a littoral condition of thedeposit. The fauna of this belt will be also varied by many stray species, eitherdriven in by storms or brought down by rivers, land shells and plants being fre-quently mixed up with its true marine inhabitants.

Professor Forbes enumerates from the Egean Sea 38 species of the Lamelli-branchiata and 107 of the Gasteropoda .

Second Region .—It extends from 2 to 10 fathoms, and the sea bottom is generallyeither sand or mud. In the Mediterranean, the Holothuriae abound in this region, asdo the burying Conchiferse. /

In the Egean this zone produced 34 species of Lamellibranchiata , and of the Gas­ teropoda 76 species, so that the number of species is considerably less than in thepreceding zone; and it may also be observed that not being entirely beyond theaction of the storm wave, its inhabitants are often washed up and carried into thelittoral zone.

Third Region — From 10 to 20 fathoms, the sea bottom being generally gravellyin places, with great tracts of sand. This zone presents few peculiarities, and is, asit were, one of transition. Large Holothuria; are still abundant,—of Lamelli­ branchiata there are 51 species in the AEgean, and of Gasteropoda 74. There is littledoubt that the extension of the muddy sea bottom of the last zone into this, bringsalso with it an approximation in the zoological characters of the two zones.