266 THE LIFE OF CRUSTACEA 



other characters, shows that they are linked on to 

 the Mysidacea, and must have arisen from some 

 primitive member of that group, like Pygocephalus. 

 Although palaeontology as yet gives little help in 

 tracing the course of their evolution, we can im- 

 agine what the intermediate links must have been 

 like by comparison with the living Cumacea and 

 Tanaidacea. 



It is possible, indeed, that the divergence of the 

 Isopod line of descent from that of the Mysidacea 

 took place earlier than the Carboniferous epoch, 

 for there has recently been discovered in rocks of 

 Devonian age in Ireland a single specimen of a fossil, 

 to which the name of Oxyuropoda has been given, 

 which has every appearance of being an Isopod. 

 At all events, undoubted Isopods make their appear- 

 ance in rocks of the Secondary Period, and some of 

 those from the Jurassic epoch are not very different 

 in general form from types still existing. 



Some of the Carboniferous shrimp-like Crustacea 

 present characters which seem to point in the direc- 

 tion of the Stomatopoda, and fossils which clearly 

 belong to that group are found in Jurassic and 

 later deposits. In the Cretaceous epoch there were 

 Stomatopoda resembling modern types so closely 

 that they have been referred to the existing genus 

 Squilla. We are even able to say that they resembled 

 the living Stomatopoda in their mode of development, 

 for larvae of the type known as Erichthus have been 



