28 



CRUSTACEA. 



this is not usual among the Decapoda. In the SquillidaB, however, 

 we End the segment pertaining to the first antennas pushed forward 

 and placed anterior to the carapax, and the first, or ophthalmic seg- 

 ment, is still more forward, or forms the anterior portion of the first 

 an ten nary segment. 



According to these explanations then, the carapax of the Brachy- 



ura, includes — 



I. The first and second normal segments, represented by the epi- 

 stomc. or its anterior portion, and the inter-antennary septum. 



II. The third normal segment, represented by the main body of 

 the carapax, and the anterior portion of the praelabial plate or palate. 



III. The fourth normal, or mandibular segment, represented by 

 the posterior and outer part of the pralabial plate, and the ventral 

 pieces of the carapax. 



It remains to speak of the areolate markings in the carapax of the 

 Brachyura, and also of the relations of the parts to those homologous 

 with them in the Macroura. 



4. Areolate Markings on the Carapax of Grabs. — The areas into 

 which the surface of the carapax of Crabs is subdivided, were in part 

 distinguished and named by Desmarest. But there is a uniformity of 

 character and number which this author did not fully recognise. 



In those species of Cancroidea, which have the markings most per- 

 fectly brought out, and which exhibit best the system of areas, the 

 carapax is divided transversly, in the first place, by a depressed 

 line, commencing just anterior to the last of the normal lateral teeth, 

 and crossing the carapax back of the middle; there is thus & poste- 

 rior and an anterior portion. 



The anterior portion is divided into three parts by depressions ex- 

 tending from back of the orbits obliquely inward to the ' transverse 

 depression alluded to, which they meet a short distance either side of 

 the centre. There are thus, to the anterior portion of the carapax, a 

 medial region, and two antero-tateral regions. The medial region covers 

 the stomach, from which it seems indirectly to derive its former out- 

 line^ and includes the gastric and genital regions of Desmarest. 



Near the front, between the orbits, a transverse line separates from 

 the medial region, a region that we may call the frontal And on 

 either side, the orbits form another region, which may be designated 

 the orbital. 



