T H A L A S S I N I D E A. 505 



the figure of Thalassina gracilis, as if a transversa articulation marked 

 the lateral pieces as well as the medial. In the species referred to, 

 there are two quite distinct sutures on the dorsal piece, and another, 

 anterior to these, more faint, appearing to divide this piece into four 

 segments. 



Such are some of the diversities of structure in the carapax of 

 the Thalassinidea. The genera Axius, Calocaris, and Laomedia, differ 

 little in these respects from Gebia. In Axius stirhynchus, there are 

 only faint traces of the dorsal longitudinal sutures, nearly as in As- 

 tacus. 



In studying out the homologies of these sutures and the areas the}- 

 bound, we may first compare the species with the structure in Pa- 

 gurus and iEglea, and we would therefore refer again to Plate 28, fig. 

 4, and Plate 32, figs. 1, 5. A general resemblance is at once apparent. 

 In figures 1 a and 1 b of the Gebia pugettensis, and 5 a, 5 b of Thalassina 

 gracilis, the transverse dorsal suture (d) is similar to that of Pagurus 

 and JEglea; it bends forward on either side to the base of the outer an- 

 tennse. The longitudinal dorsal sutures {p) are also similar, and 

 either correspond to p or I. In Gebia, the suture n is distinct, and 

 the piece S of Pagurus and iEglea, with the space below, has its 

 analogue. In Thalassina, the suture n is also distinct, though 

 losing itself below, as in Pagurus, and the position of it differs from 

 Gebia, only in resembling Pagurus more closely, although the area >S 

 is not distinct. It is hence obvious that the structure of the carapax 

 in the Thalassinidea is essentially the same as in Pagurus and iEglea. 



With regard to the relations of these portions of the carapax, or of 

 the sutures separating them, to the Brachyural structure or the normal 

 series of segments, we offer the following considerations, in addition to 

 those presented on page 32 and the following. The portion of the 

 carapax pertaining to the second antennary segment must be the 

 anterior dorsal portion, if the distinction exists. This part covers or 

 encloses the antennary portion of the front, and is circumscribed by a 

 strong suture. It seems, therefore, to be normally this segment and 

 the whole of it. If the rest of the carapax is to be considered the 

 epimeral portion of this segment, what are we to say of its dividing 

 sutures? for they seem to show that this posterior part contains a 

 medial and two epimeral pieces of its own, as if normally an inde- 

 pendent segment. Again, the antero-lateral piece in Gebia passes 

 with a broad surface into the postero-dorsal, a long part of the longi- 



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