390 



ARTHROPODA. 



to eight pairs of legs, and the branchiae are not enclosed in a 

 cavity beneath the thorax, but are either suspended beneath the 

 abdomen, or, more rarely, attached to the thoracic legs. 



Of the living Stomapods the best-known forms are the 

 Locust-shrimps (Squilld), the Glass-shrimps (Erichlhys), and 

 the Opossum-shrimps (My sis). 



As regards fossil Stomapoda, considerable uncertainty still 

 exists, owing to the imperfect condition of preservation of 

 many extinct Crustaceans. The most 

 ancient type which has been usually 

 referred here is the Palceocaris typus 

 (fig. 245), of the Coal-measures of North 

 America. This form is clearly related to 

 G-ampsonyx, and if it shall be shown to 

 possess sessile eyes, it must be removed 

 to the Isopoda or Amphipoda. Here, 

 however, we shall regard it as an early 

 and " comprehensive " type of the Podoph- 

 thalmata, characterised by the persistent 

 segmentation of the thorax, but in other 

 respects presenting considerable resem- 

 blance to the Macrurous Decapods. It 

 differs from Gampsonyx in various points, 

 but especially in the fact that the legs 

 are undivided, whereas in the latter genus 

 they are bifid. The Pygocephalus of the 

 Coal-measures appears also to be a Sto- 

 mapod. In deposits later than the Car- 

 boniferous, Stomapodous Crustaceans have 

 been occasionally detected, but the genus Squilla itself does 

 not appear to be represented in rocks older than the Eocene 

 Tertiary. 



OEDER DECAPODA. 



Crustaceans with Jive pairs of ambulatory legs, of which the 

 first pair is modified to form nipping-claws, some of the other 

 pairs behind this being often chelate as well. There is a large 

 cephalothoracic carapace, and the branchice are contained in 

 cavities at the sides of the thorax. 



Fig. 245. Palceocaris 

 typus, slightly enlarged. 

 From the Coal - measures 

 of North America. (After 

 Meek and Worthen.) 



