504 CRUSTACEA. 



cends obliquely on leaving the front margin ; while in Callia- 

 nassa it is nearly horizontal. Hence, in a side profile view of the 

 antero-dorsal portion of Gebia, its form is triangular, being highest at 

 middle; while in Callianassa, the height is small and nearly equal 

 throughout. At first sight, it seems as if the oblique suture of Gebia 

 is wanting; but on consideration, it appears evident, that it only has 

 a different position, and in each there is the same termination of the 

 suture near the base of the outer antennae. The same difference 

 exists between Cenobita and Pagurus. 



The sutures, thus far alluded to, are all that exist in Callianassa. 

 But in other genera there are additional sutures, subdividing either 

 the lateral pieces, or the postero-dorsal, or both. In Gebia and Tha- 

 lassina there is a strong suture, sometimes the strongest or most open 

 in the carapax, dividing vertically with a curve the lateral pieces into 

 an anterior and a, posterior portion. In Thalassina, this suture descends 

 from near the transverse dorsal suture ; in Gebia, it has a more poste- 

 rior position, a large interval separating it from this dorsal suture ; its 

 direction is also more oblique, bending more forward, as it descends 

 with a curve towards the lower margin, which it hardly reaches (see 

 Plate 32). In Thalassina, this suture is very open, except at a 

 single point of coalescence, and the edge of the posterior portion is 

 dentate or denticulate; although so apparent, it is lost before reaching 

 the lower margin. The point which it would reach if extended is 

 normally very near the same as in Gebia. 



This suture hence divides the lateral pieces into an antero-lateral 

 and postero-lateral ; the antero-lateral is oblique from above down- 

 ward and forward in Gebia, and from above downward and backward 

 in Thalassina. 



The antero-lateral piece is not farther subdivided in Thalassina, or 

 not distinctly so. But in Gebia, it is crossed often by a horizontal 

 suture, a little oblique, which divides it into a lower and upper (or 

 marginal and inner) portion. This suture is seen in the lateral vie\v 

 of Gebia hirtifrons (pi. 32, fig. 2) . 



The postero-dorsal piece is broad, and without any subdivisions in 

 all the genera excepting Thalassina. In this genus its form is narrow, 

 becoming narrower behind and terminating in a point. Moreover 

 in its posterior part it is crossed by two or three sutures looking some- 

 thing like obsolete articulations. A corresponding suture sometimes 

 occurs for a short distance in the lateral piece adjoining, as seen in 



