422 SCIENCE PROGRESS 



habit of secreting lime ^ in the exoskeleton (which would thus 

 become purely chitinous) concomitantly with their migration 

 from the sea into fresh water. The very fact of a creature with 

 a calcareous exoskeleton taking to a fresh-water existence 

 necessarily implies the diminution or total abandonment of the 

 calcareous portion, owing to the solvent action of the relatively 

 large amount of carbon-dioxide in river-water. The massive, 

 stony carapace of most lobsters and crabs is not to be found 

 among their fresh-water relatives such as the crayfish or the 

 river-crab (Thelphusa). In the latter forms the carapace, 

 though still in part calcareous, is so thin that it can be cut 

 with a pair of scissors. The erosion of the umbos or apices 

 of fresh-water molluscs, where the protecting epidermis has 

 been worn away, is a familiar instance of the destructive power 

 of carbon-dioxide on calcareous shells. According to Semper- 

 the epidermis becomes destroyed in the first place in these 

 prominent parts of the shell by boring fungi aided by the 

 wearing action of fresh-water currents. 



As a matter of fact, the young states of Blattidce {e.g. of 

 the Australian Oniscosoma among living insects and of some 

 of the Carboniferous insects, v. infra) show that the wings 

 are actually nothing but lateral expansions of the segments 

 exactly as the pleurae of Trilobites are the lateral expansions 

 of the body-segments. It is only necessary to presume that 

 the pleurae of the hypothetical fresh-water Trilobite became 

 thin and membranous and traversed by tracheae in order to 

 obtain an exact counterpart of a tracheal gill. 



It is a matter of considerable importance that no undoubted 

 insect-remains are known previously to the Carboniferous 

 epoch, for the Protocimex silurica of the Upper Ordovician 

 of Sweden and the Palceoblattina Douvillei of the Silurian of 

 Calvados are evidently merely inorganic structures. Further- 

 more, the Little River group of New Brunswick, which has 

 yielded so many remains of insects and was formerly con- 

 sidered to be of Upper Devonian age, has now been clearly 

 proved to be not older than the lower part and not higher 

 than the middle part of the Upper Carboniferous. Hence we 



' It must be noted, however, that in the IMyriapod DiPLOPODA (Chilognatha) 

 out not in other IMyriapods the chitinous exoskeleton contains calcareous matter 

 and is thus rendered much less flexible than in the other orders of Myriapoda. 



- Natural Conditions of Existence., etc. p. 213, London, 1881. 



