506 CRUSTACEA. 



tudinal suture separating them. In Thalassina, this union seems to 

 be almost intercepted, yet there is a narrow connexion which is un- 

 broken by a suture. In each, therefore, the antero-lateral piece and 

 the postero-dorsal (or, at least, its anterior part) have a mutual de- 

 pendence; and the absence of a suture between in one case, and its 

 indistinctness in the other, compared with the transverse dorsal suture, 

 show that this dependence is of the closest kind, even that of parts of 

 a common normal segment; moreover, the postero-lateral piece also is 

 probably a part of the same. We may, therefore, conclude, that the 

 anterior and posterior divisions of the carapax in the Thalassinidea, 

 are normally distinct segments ; and they correspond, the first to the 

 second antennary normal segment, and the second to the mandibular. 

 This conclusion, in all its extent, should not be taken as universal 

 for the Macroura, or even for the Thalassinidea. It has been shown 

 among the Entomostraca, that, while the carapax has similar relations 

 to those of the Brachyura in some genera, in others, closely allied as 

 the Cyclopacea, it undergoes a subdivision into segments. And this 

 subdivision depends partly on the connexion of the carapax with the 

 thorax beneath. The whole carapax, as in Cypris, may normally 

 pertain to one or two cephalic segments ; again, some of the posterior 

 of the thoracic segments, as in Cyclops, may so unite in constituting 

 it, or as it were come to the surface dorsally, that the articulations of 

 these segments become apparent. This last fact may afford the true 

 explanation of the transverse sutures in the postero-dorsal piece in 

 Thalassina. The normal segments below, here seem actually to con- 

 tribute to the carapax ; and the sutures corresponding in the postero- 

 lateral portion of the carapax appear to indicate the same articulations 

 or segments, and properly their epimeral portions. The correspon- 

 dence is so close between these posterior subdivisions and the articula- 

 tions in the Entomostraca, referred to, that we cannot fail to see in 

 both, exemplifications of a single general law. The Thalassinidea, 

 however, afford us a good ground for a general conclusion with regard 

 to the other Macroura. This is evident from the structure of the 

 carapax in Astacus, a genus more closely related than any other to 

 the Thalassinidea. There is in this genus a transverse dorsal suture, 

 which is very distinct. But besides this, there are traces of the longi- 

 tudinal sutures of the posterior part of the carapax, as shown in the 

 figures, on Plate 33 ; and these sutures are separated sometimes by a 

 narrow linear area, as in Thalassina, and sometimes by a broader area, 



