THORACOSTRACA OR PODOPHTHALMATA. 



563 



cephalothorax, and its posterior extremity is in the form of a 

 powerful natatory organ or caudal fin. The recent Macrurans are 

 mostly marine in habit, but some typical forms (the Cray-fishes) are 

 inhabitants of fresh water. The earliest fossil representatives of the 

 Macrura are certain Shrimp-like Crustaceans which appear in the 

 Devonian and Carboniferous rocks, but the group undergoes a great 

 development in the Mesozoic period, and many Tertiary forms are 

 known as well. It will be suf- 

 ficient here to give a general 

 sketch of the geological history 

 of the group, without discussing 

 its different families in any de- 

 tailed manner. 



The oldest known Macruran 

 is the Shrimp-like Palceopalcz- 

 mon of the Devonian rocks of 

 North America, the general 

 characters of which would jus- 

 tify its reference to the exist- 

 ing family of the Shrimps and 

 Prawns (Candida). To the 

 same family may be referred 

 the Carboniferous genera An- 

 thrapalcemon (fig. 425), Cran- 

 gopsis, and Pygocephalus. In 

 Anthrapalcemon^ there is a 

 well-developed carapace, which 

 is furnished with a beak or 

 "rostrum," and possesses ser- 

 rated lateral margins. There 

 are five pairs of thoracic legs ; 

 the abdomen is composed of . J 



c , , Fig. 424. — The common Cray-fish (Astacusfluvi- 



tree Segments, and there IS a otitis), viewed from below, a, Antennules; b, 

 rmirhl fin fnrmprl hv thp tpl Antennae; c, Eyes; d, Opening of antennary 



cauaai nn, iormea Dy me tei- gland . ^ Last pair ff 00 t-jaws;./; one of the great 



SOn along with the last pair Of chelae ; g, Fifth thoracic limb ; h, Swimmerets ; i 

 ° x Ine last pair of swimmerets; j, Ihe opening of 



SWimmeretS. No Undoubted the anus below the telson. 



representatives of the Macrura 



have hitherto been detected in the Permian rocks, but the Trias 

 has yielded a number of Long-tailed Decapods belonging to the 

 families of the Shrimps (Penceus and s£ger), the Eryonidce, and the 

 Glyphceidce (Pei7iphix, Lithogaster, &c.) It is, however, in the 

 Jurassic rocks that the Macrura attain their maximum develop- 

 ment as fossils, all the recent families of the order having now 

 come into existence, and some of them attaining here their cul- 

 minating point. The Shrimps (Carididce) are represented by genera 



