624 CRUSTACEA. 



Largest specimen in the collections, three and a half inches in 

 length. The colour is pale yellowish, with veinings and blotches of 

 bright green. 



Squilla chiragra, FABR., DESMAREST, Crust., 251, pi. 43. 



G. chiragra, LATR., Encyc. Meth., x. 473, pi. 325, f. 2 ; EDWARDS, Crust., ii. 528. 



FAMILY II. ERICHTHID^E. 



GENUS ERICHTHUS. 



THE prominent distinction laid down by Milne Edwards for distin- 

 guishing Alima and Erichthus, is the different length of the carapax; in 

 the former, the carapax covering only part of the thorax, while in the 

 latter, it covers the whole thorax and one or more abdominal segments. 

 But both this character and that based upon whether the ophthal- 

 mic and antennary segments are under the beak or not, afford no 

 good distinction. The third species of Erichthus here described (E. 

 spiniger) has the carapax shorter than the thorax, and, therefore, is an 

 Alima, according to the accepted characteristic. Yet it more closely 

 approaches Erichthus in its general stout form. Moreover, it has the 

 anterior part of the cephalothorax preceding the mouth shorter than 

 the following portion, quite unlike the true Alimae. We have there- 

 fore based our characteristic more upon the length of the anterior 

 portion of the cephalothorax, which is certainly of more importance 

 than the length of the carapax, an appendage that varies much in 

 length in many genera, and especially in the tribe under consideration. 

 The form of the anterior part of the cephalothorax is very long and 

 slender in Alima, and short in Erichthus, much longer than the part 

 posterior to the mouth in the former, and much shorter in the latter. 

 We believe this course more true to nature, than the instituting a 

 new genus "Alimerichthus." 



The species of Erichthus differ much with respect to the spines of 

 the carapax. The following is a list of those known : 



