ANCEUS HALIDAII. 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 bya 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. mazillaris, 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 
