ON THE BRACHYUROUS DECAPODS OF THE CAPE. ' 55 



the following descriptions I shall rarely allude to colour, since experience has taught me that in 

 this respect dried specimens of Crustacea are not to be trusted. I would also observe, that as the 

 groupesof degree next inferior to families — namely, genera — have never been worked out in this 

 class, I have for the present considered almost all the various groupes under families to be sub- 

 genera, although some of them, such as Plagusia, may possibly be a true genus, and others again 

 may prove to be only sections of some sub-genus. I do not think that our collections, as yet, 

 possess a sufficient quantity of species to allow us to decide what are the genera and what are 

 the sub-genera of Decapod Crustacea. Professor Dehaan, however, has offered some valuable 

 hints on the subject, and to those I refer the reader. We appear to enter among the 

 Brachyura by the genus Mycteris, and to leave it by means of the genus Ranina. There are 

 ten Brachyurous stirpes, which may be placed in two columns, so as to shew those analogies 

 which by Milne Edwards and others have too often been taken for affinities. 



TETRAGONOSTOMA. Analogies. trigonostoma. 



Mycteris ..PiNNOTHERiNA. Shell orbicular. Dromiina — Ranina. 



Grapsina. Shell quadrilateral. Dorippina. 



Cancrina. Shell arcuated with feet often natatory. Corystina. 



Parthenopina. SheU uneven with crested feet. Calappina. 



Inachina. Shell sub -triangular and generally spined. Leucosina. 



3. The analogy between certain hiachina, such as Acantlionyx, and certain Leucosina, such 

 as Nursia, is so great as to have induced M. Latreille to imagine that a direct affinity existed 

 between the two groupes. In general, the above analogies appear reversed ; but the Tetra- 

 gonostomous stirpes may be characterized as follows : — 



Tribe TETRAGONOSTOMA. 



stirpes* 



Normal Groupe. 



OXYRHYNCHA, M. E. 



Epistome veiy large. Cly-^ 

 pens generally advanced in 

 front. 



Aberrant Groupe. 



Brachyrhyncha. 

 Epistome short, Clypeus 

 rarely advanced in front. 



1 Inachina. 



Triangular Crabs. 



2 Parthenopina. 



Rocky Crabs. 



3 Cancrina. 



Arched Crabs. 



4 Grapsina. 



Square Crabs. 



5 PiNNOTHERiNA. 



Parasitical Crabs. 



r First joint of external antennae very large, forming the 

 J greater part of the lower side of the orbit, and always 



soldered to the clypeus. 

 fFirst joint of external antennse small, not soldered to the 

 ^ clypeus and not aiding to form the lower side of the 

 orbit of the eye. 



rXigellus of external pedipalps always inserted at the inner 

 J angle of their third joint. The scapes of the palpi 

 I unidentated on the inside. Shell arched in front, 

 f Tigellus of external pedipalps inserted at the outer angle, 

 or at the middle of the third joint. The scapes of 

 palpi not dentated. SheU quadrilateral. 

 Tigellus of external pedipalps always inserted at the sum- 

 mit, or at the outer angle of their third joint. The 

 scapes of the palpi not dentated. Shell orbicular. 



( 



1 



4. By Eurynome we pass from the Inachina to the Parthenopina ; by means of jEthra we pass 

 from the Parthenopina to the Cancrina ; from these to the Grapsina by Thelphusa ; from the 

 Grapsina to the Pinnotherina by means of JDoto ; and from the Pinnotherina we return 

 to the Inachina by means of Elamene. The following appear to be the famiHes of Ina- 



