CRUSTACEA. 



175 



ception, however, no Limuloid Crustaceans are known till the 

 Carboniferous Rocks are reached. Here we have the two 

 genera Prestwichia and Belimirns, the former represented by 

 two, the latter by four species. In Prestwichia (fig. 119), the 

 thoracic and abdominal segments are not separable from one 

 another, and the former 

 are anchylosed or amal- 

 gamated, as well as the 

 latter. In Belinurus there 

 are five thoracic and three 

 abdominal segments (as 

 in the preceding), but the 

 thoracic somites are free 

 and movable, whilst the 

 abdominal ones are an- 

 chylosed. The only other 

 genus of the Xiphosura 

 is {Limulud itself. This 

 genus is represented by 

 forms doubtfully here re- 

 ferable as early as the 

 Permian Rocks. An- 

 other dubious form oc- 

 curs in the Trias. Seven 



species have been described from the Lithographic slates of 

 Solenhofen (Middle Oolites). One doubtful form occurs in 

 the Chalk, a single Tertiary species has been described, and 

 four species are known as existing at the present day. 



Fig. 119. Prestwichia (Limulus) rotundata, 

 Coal-Measures. 



CHAPTER XVI. 



CR USTA CE A Concluded. 

 MALACOSTRACA. 



THE Malacostracous Crustaceans are distinguished by the 

 possession of a definite number of body-segments, seven 

 somites generally going to make up the thorax, and an equal 

 number entering into the composition of the abdomen (count- 

 ing the telson as a somite). The Malacostraca are divided 

 into two primary sections, termed respectively Edriophthal- 



