300 Transactions. — Zoology. 



The bonudary between the two coxocerites is marked externally by 

 a short median longitudinal groove ; the point of a fine scalpel pushed into 

 this is found to penetrate easily a short distance into the interior of the 

 head. When the soft parts are removed, and the head is examined from 

 within, the instrument is found to have passed into an extremely narrow 

 cleft between two delicate closely applied plates (i.ex. pi), which together 

 form a median crest projecting upwards into the cavity of the head. These 

 plates evidently represent a small portion of the applied inner walls of the 

 coxocerites, and may be called the internal coxoceritic, or, for shortness, coxal 

 plates. The posterior and outer edge of each coxocerite is similarly pro- 

 duced into a vertical obliquely-set plate {e.cx. pi), which may be called the 

 external coxal plate. It is very possibly formed, in part, by the epistoma. 



The basicerite (be), or proximal segment of the movable part of the 

 antenna, articulates with its socket (ant. 2') in the usual way by two hinges ; 

 one of these is dorsal and internal (fig. 4, h), and is situated at the upper 

 end of the antennulary sternum just where it turns sharply backwards ; the 

 other is ventral and external, and is situated at the junction of the coxocerite 

 with the carapace. The articulation is thus an oblique one, elevation being 

 accompanied by abduction, depression by adduction. 



The rule in Crustacea is for each segment of a limb to be moved by two 

 muscles, inserted into its proximal edge at points intermediate between the 

 hinges. There are two such muscles to the basicerite of Palinurus edwardsii: 

 one inserted dorso-externally, arising from the inner surface of the carapace 

 and acting as combined elevator and abductor ; the other ventro-internal, 

 arising bi-pinnately from the outer surface of the internal, and from the 

 inner surface of the external coxal plate, and acting as combined depressor 

 and abductor. But besides these, the proximal edge of the basicerite is 

 produced, immediately below the insertion of the elevator, into a strong 

 serrated ridge, into which is inserted a muscle arising from the outer face 

 of the external coxal plate. This being inserted above and external to the 

 ventro-external hinge, must act as an additional elevator and abductor. 



Essentially the same structure of the anterior part of the head is seen in 

 P. lalandii and in P. hugelii : in the latter species, as I am informed by 

 Mr. T. W. Kirk, the clasping processes are smaller than in P. edwardsii, but 

 there is no difference of importance. 



In P. vulgaris, on the other hand, great and important differences are at 

 once apparent. A vertical section (fig. 2) shows that the rostrum (r) is a 

 mere insignificant spine, smaller than those on the lower edge of the supra- 

 orbital spines (s. or. s?>), and no larger than the spiniform tubercles with 



