170 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



b. The posterior margin of the carapace is divided into 

 a median and two lateral portions. It carries no large 

 serrations, but a very finely dentated ridge runs parallel 

 to and very near its edge. 



c. The dorsal surface of the carapace is mapped out by 

 depressions into several areas. 



1. On the dorsal median line, somewhat nearer the pos- 

 terior than the anterior end, there is a transverse depres- 

 sion, about half an inch long, the outer ends of which 

 unite, at an obtuse angle, with two straight depressions, 

 which run forwards and outwards, to unite anteriorly with 

 two lines which run outwards on to the horns at the lateral 

 angles of the carapace. 



This system of depressions appears to be homologous 

 with the cervical suture of the crayfish or lobster, and 

 divides the carapace into an anterior cephalic, and a poste- 

 rior thoracic area. 



(i.) The thoracic area is again divided, by a pair of 

 faintly marked depressions, running from the outer ends 

 of the transverse bar of the cervical suture to the edges 

 of the carapace, over the last pair of legs, into a central 

 cardiac, and tw r o lateral branchial areas. 



(ii.) The cephalic area is divided into the following 

 regions : 



(.) An irregular transverse depression, crossing the 

 middle of the carapace near its anterior edge, and bend- 

 ing forward at its ends to meet the anterior edge over the 

 eyes, marks of an anterior or facial region, which is again 

 divided into a nunY\un frontal lobe, and two orbital lobes. 



(6.) The space between the facial depression and the 

 cervical suture is divided by two longitudinal furrows into 

 a large, median, sub-triangular, gastric region, and two 

 hepatic lobes. 



