T H A L A S S I N I D E A. 593 



Subtribe I. THALASSINIDEA. 



The group Thalassinidea is remarkable for the wide limits in rank 

 covered by the species. As has been observed, they fill up an inter- 

 val between the Anomoura and Squillidea, forming an almost com- 

 plete line, independent of the other Macroura ; they inosculate with 

 the latter (particularly with the Astacidae) in a few species only, 

 without entering into a common series with them. There is great 

 diversity in the legs, the branchiae, the maxillipeds, and more remark- 

 ably still, in the carapax, and the abdomen with its appendages. We 

 dwell here upon the peculiarities of the carapax alone, as the other 

 points have been more fully developed by other authors. 



In some species, as those of the genus Gebia, the head has an un- 

 usual vertical height, as seen in a profile view ; the outer antennae 

 are placed low and the base is in view, very much as in Cenobita ; 

 indeed, a comparison brings out a very close similarity to that genus 

 in the position of these parts. In others, as Callianassa and Thalas- 

 sina, the outer antennae have a much higher position, being nearer 

 the beak ; and the base is covered by the expanded side of the cara- 

 pax. In this, we have a resemblance to the genus Pagurus, which 

 differs in this way from Cenobita. The figures on Plate 32 exhibit 

 well these peculiarities. 



The subdivision of the carapax by sutures, is strikingly diverse in 

 different genera, although conformed to a single system. There is 

 commonly a transverse dorsal suture, and either side of the dorsal 

 portion of the carapax, a longitudinal suture, more or less distinct. 

 The transverse suture passes either side into the dorsal, and partly or 

 wholly follows it to the anterior margin of the carapax. The two 

 sets of sutures together, hence divide the carapax into four parts or 

 areas, a dorsal anterior, a dorsal posterior, and two lateral. 



Let us now examine into the forms of these areas, and the subdivi- 

 sions they undergo. 



The longitudinal suture either side commences at the base of the 

 outer antennae, and extends to the back margin of the shell; the 

 position of the base pf the antennae will therefore modify somewhat 

 the direction of the longitudinal sutures. In Gebia, where the inser- 

 tion of these antennae is at an unusually low level, this suture as- 



