GRAPSOIDEA. 307 



straight or arcuate sides, and a wide male abdomen, very nearly 

 covering at its base, with few exceptions, the whole breadth of the 

 sternum. The few exceptions with respect to the abdomen, here 

 alluded to, are among species in which the short eyes, depressed form, 

 and margined sides of the carapax are so distinct, that they could not, 

 by any mistake, be referred to another family among the Grapsoidea. 

 The genus Helice alone is ambiguous in its relations, being somewhat 

 related to Cleistostoma ; and through these groups the Macrophthal- 

 midse and Grapsidse pass into one another ; but both this genus and 

 Chasmagnathus have the oblique piliferous crest upon the outer max- 

 illipeds which is so characteristic of the Sesarma group, a peculiarity 

 that does not occur among the true Macrophthalmidae. 



The fourth family is the Gecarcinidjs — distinguished, as with other 

 authors, by having an inflated form, and high, broadly-rounded sides, 

 instead of anterior angles and a dentated lateral margin. The abdo- 

 men at base, as in most of the Grapsidse, covers the whole breadth of 

 the sternum. While all the Grapsidse have the ridge on the praela- 

 bial plate or palate, bounding the efferent channel, very prominent, 

 the Gecarcinidse have no such rid^e. 



The fifth family is the Pinnotheridse. Like the last family, the 

 body has rounded sides, but the form may be very much depressed, 

 or much inflated. The male abdomen is very narrow, being much 

 narrower at base than the corresponding part of the sternum. The 

 genus Pinnotherelia forms a transition to the Gecarcinidse, in having 

 a rather broad front between the eyes, and the antennary fossettes 

 separate. Some of the Pinnotheridse have the second joint of the 

 outer maxillipeds obsolete or obsolescent ; but there are very gradual 

 transitions among the species, to those in which the form is nearly the 

 same as in certain of the Gecarcinidse, with the second joint larger 

 than the third. The eyes have regular orbits, but in some species 

 related to Hymenosoma, the eyes are not retractile into orbits, 

 any more than in Pericera among the Maioid Crustacea. A few 

 species have a somewhat triangulate and subrostrate form, resembling 

 the Inachidse, with which group they were associated by early authors. 

 This is most striking in the genus Elamena. But even here, the 

 male abdomen and male sexual appendages have in all respects a 

 Grapsoid character. 



The sixth family is the Myctirid^e. The genus Myctiris is near 

 the Pinnotheridse in general form ; but it has no distinct orbits. The 



