13 



by a well-defined suture, forms a septum* between the 

 articular cavities of the two optic peduncles. 



From the dorsal side of the sternal region the septum 

 between the above-mentioned articular cavities is short 

 and broad. On a level with the posterior end of these 

 cavities there is a well-marked suture separating the 

 septum from the first sternum. 



Immediately in front of the dorsal side of the 

 ophthalmic articular cavities (o.nt.c.) are two short 

 calcareous plates near the median line, which stretch 

 across to the roof of the carapace. These are the 

 Pi'ocephalic processes (p.c.p.) to which are attached the 

 anterior gastric muscles. 



The First Sternum (S l ) lies in the segment of the first 

 antennae (ant le .} and separates the articular cavities of 

 these appendages. Owing to the depth of the sternum in 

 this region its relationship to the articular cavities is best 

 seen from the dorsal side of the sternum. 



It consists of a median piece lying between the 

 articular cavities of the antennules, and of two lateral 

 expansions which form the posterior boundaries of the 

 articular cavities. Viewed laterally the sternum is seen 

 to have a comparatively great depth. About half way 

 down the anterior face of this sternum is a concavity into 

 which fits a process from the septum between the 

 ophthalmic articular cavities. 



From the ventral side the first sternum bounds the 

 posterior and inner sides of the sockets of the antennules, 

 and the lateral prolongations extend as far as the bases of 

 the second antennae. 



* In the present Memoir the optic peduncles are not regarded 

 as modified appendages, and I shall not regard the region of the body 

 from which the eyes arise as the first segment, nor shall I speak of 

 the septum between the articular cavities of the optic peduncles as 

 the first sternum. 



