CRUSTACEA. 



177 



under the name of Prcearcturus. It is believed to resemble 



the living Arcturus Baffinsii. From the Carboniferous Rocks 



an Isopod has been described under 



the name of Acanthotelson. In the 



Permian Rocks we have the genus 



Prosoponiscus, which, however, has 



been referred to the Amphipoda. 



In the Upper Oolites (Purbeck 



beds) occurs the Arch&oniscus 



Brodiei (fig. 120), often in large 



numbers. In the Chalk occurs the 



genus Palaga, which ranges to the 



Fig. >.Arch<roniscus 

 Miocene Tertiary. Lastly, several fossil isopod, from the Upper 



J . > ' Oolites. 



forms, some of which are of very 



uncertain affinities, have been described from the Tertiary 



Rocks. 



DIVISION B. PODOPHTHALMATA. The members of this 

 division are Malacostracous Crustaceans, in which the eyes 

 are compound, and are supported upon movable stalks or 

 peduncles, and the anterior portion of the body (cephalo- 

 thorax) is protected by a carapace. In this division are in- 

 cluded the two orders of the Stomapoda and Decapoda. 



ORDER STOMAPODA. 



Stalk-eyed Crustaceans in which there are generally from six 

 to eight pairs of legs, and the branchm 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 (Squilla), the Glass-shrimps (Erichthys], and 

 the Opossum-shrimps (Mysis). The earliest-known example 

 of the Stomapoda is the Gampsonyx fimbriatus of the Carboni- 

 ferous Rocks ; and the Pygocephalus Couperi of the same 

 formation is also probably to be referred to this order. The 

 genus Squilla itself does not appear to be represented in rocks 

 older than the Eocene Tertiary. 



ORDER 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 branchm are contained in cavi- 

 ties at the sides of the thorax. 



M 



