ANCEUS HAL1DAII. 205 



very short transverse ridge, and the two succeeding seg- 

 ments are much shorter than the three posterior, which 

 are irregular in the form of their lateral margins, the 

 posterior angles of each of which is produced into a 

 rounded lobe. The tail (pleon) is composed of six seg- 

 ments, of which the five anterior are furnished on each 

 side with a double scale, the outer division being narrower 

 than the inner, and terminated by a slender point, or short 

 strong seta, whilst the inner division is oval, slightly 

 ciliated at its margins, of a delicate transparent texture, 

 allowing the circulation to be seen within, and which is 

 carried round the organs within the lateral margins, as 

 indicated by the arrows in the accompanying figure ; the 

 tail is terminated by a small triangular joint, furnished 

 at each side with a pair of strongly ciliated, delicate 

 elongate-ovate plates. The legs are more robust than 

 in A. maxittaris, and have the terminal joints armed with 

 several small spines. 



The female closely resembles that of A. mazillaris, 

 but is at once distinguished by the form of the outer 

 appendages of the mouth (the head of this sex is repre- 

 sented in the upper right-hand figure of the engraving 

 upon page 190). These organs are composed of five 

 joints, of which the basal one is the largest, and is 

 attached near the outer posterior angles of the underside 

 of the head in a transverse direction, the remaining 

 joints forming a somewhat oval flattened mass, of which 

 the first joint is small and transverse, the next larger and 

 semi-ovate, and the terminal joint very minute. We 

 have not been able to discover any trace of the inner 

 pair of foot-jaws. 



This species was discovered by our friend Mr. Alex- 

 ander H. Haliday, and is referred to in a communication 

 dated October 9, 1847, which was published in the 



