XCii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1 896, 



Crustacean class. Is this then the great barrier-reef between the 

 Palaeozoic and Neozoic life-periods ? Do we indeed find here the 

 beginning of all modern forms of Crustacea and the ending of all 

 ancient ones ? — By no means ; nor is there any period in the whole 

 geological record at which a hard-and-fast line can be drawn 

 dividing the earlier from the later members of any group. 



Certain tribes, such as the Entomostraca, are represented through- 

 out. Others, like the Amphipoda, may perhaps extend into Silurian 

 times, — whilst Isopods and Decapods are probably represented as 

 far back as in Devonian strata. 



The Crustacea, in fact, bear testimony to the same general bio- 

 logical law which holds good in so many other classes of organisms, 

 namely, that before one order dies out and disappears, other and 

 successive groups have already made their appearance : the one 

 overlapping the other in time. 



There are, indeed, no sharp divisions in living Nature, but 

 rather a subtle interblending of groups which, like the prismatic 

 colours in the rainbow, shade off imperceptibly the one into the 

 other. 



Malacostkaca. — Throughout the sub-class Malacosteaca, to which 

 so large a proportion of the Mesozoic, Cainozoic, and Recent forms of 

 Crustacea belong, the number of segments present is generally very 

 persistent for the three divisions of the body (head 6, thorax 8, and 

 abdomen 7=21). They fall naturally into two groups, (1) the Edeio- 

 phthalma, in which the eyes are sessile, and (2) the Podophthalma, 

 in which they are pedunculated. 



Amphipoda. — In the Amphipoda belonging to the first of these 

 divisions, with sessile eyes, the body is pretty constantly and 

 regularly developed, the small cephalothoracic head-shield only 

 covers its own series of seven paired appendages, while the thorax 

 and abdomen have usually each their proper and normal series of 

 seven free somites, with a corresponding pair of limbs attached to 

 each segment. 



Most of the members of this order are of small size, with a 

 laterally compressed body ; but their numbers are almost incredible, 

 and they are most widely distributed not only in freshwater lakes 

 and rivers, springs and subterranean watercourses, but they are 

 found living along the shores of almost every land in the open air 1 

 and also between high and low tides. 



1 Some species of Orchestia are known (as 0. tahitiensis, 0. telluris, 0. sylvi- 

 cola) which live far removed from the sea and at elevations of 1000 feet. 



