CLASS inCATION OE THE MAIOTD CRUSTACEA. 635 



publication, in 183-1<, of Milne-Edwards's first volume of the 

 'Histoire naturelle des Crustaces,' wherein 30 genera of this 

 group are enumerated. 



Dana, in 1852, in liia account of the Crustacea of the U.S. Ex- 

 ploring Expedition, gives a synopsis of the then known genera, 

 the number of which had considerably increased. Since that 

 time, however, no further revision has appeared ; but during the 

 twenty-six years that have elapsed, a very considerable number 

 of new forms have been made known to science, through the 

 labours of modern carcinologists, among whom the late Dr. 

 Stimpson and M. Alphonse Milne-Edwards must be particularly 

 mentioned. The total number of well-established genera included 

 in the present revision is 106 ; but not a few of those previously 

 described are reduced to the rank of subgenera or are regarded as 

 syuonyma, and others, which are insuificiently known to me, are 

 referred to parenthetically. 



The Oxyrhyncha, as defined by M. Milne-Edwards, constitute 

 as a whole a natural group ; but no single character can be men- 

 tioned which will serve to distinguish them universally from the 

 other Brachyura. Externally they are distinguished by their 

 more or less elongated carapace (which is usually provided with 

 a rostrum and narrows anteriorly), large epistoma, longitudinal 

 antennules, and the position of the basal antennal joint, which in 

 the typical Maiidse is situated beneath the eyes. The buccal 

 cavity is quadrate, with its anterior margin straight. The 

 branchiae are nine on each side, the afterent -canal opens behind 

 the pterygostomian regions in front of the anterior legs, and the 

 efferent canal at the sides of the buccal cavity. The male 

 genital appendages arise from the bases of the fifth ambulatory 

 legs. 



Erom the Oxystomata, which are closely related to the Oxy- 

 rhyncha in the narrowness of the frontal region and the concen- 

 tration of the oi'gans of sense, the latter are distinguished by the 

 triangulate buccal cavity and the position of the afferent branchial 

 channel ; but the genus MesorJicea, recently described by Stimpson, 

 evinces a remarkable approximation on the part of the Partheno- 

 pidse to the Oxystomatous type. Erom the Cyclometopa (Can- 

 croid Crabs) the typical Maiidse are distinguished by the longi- 

 tudinal antennules and the position of the basal antennal joint ; 

 but the Earthen opida), again, occupy in this respect a position 

 almost intermediate between the rest of the Oxyrhyncha and 



