8 Mr. J. N. Halbcrt on Irish Ilydrachnida. 



Arrhenurus octagonus^ sp. n. (PI. II. fig. 5.) 



Male. — Colour during life red, with ill-defined darker 

 markings on the back. In dorsal view the body is roughly 

 octagonal in shape, the posterior half being somewhat similar 

 in outline to the anterior. Front margin almost straight, 

 about equal to half the width of the bod}' j all other margins 

 very slightly cmarginate. There are no conspicuous dorsal 

 humps, but in the middle of the posterior margin there is a 

 deep excavation, witli a prominence on each side on which 

 is a long hair. Dorsal furrow roughly circular in form, 

 enclosing a comparatively small area (length 537 yu.) of the 

 middle of the back. 



The aj)pendage is short, measuring about a sixth of the 

 entire length of the animal and about four fifths as broad ; in 

 dorsal view mostly covered by the hinder part of the main 

 body ; sides of the appendage hardly constricted at base, 

 gradually narrowing inwards and blending with the hinder 

 margin. Posterior dorsal margin with a wide excavation 

 reaching from side to side; posterior ventral margin slightly 

 sinuate, pierced in the middle by a narrow deep indentation, 

 which widens noticeably at its deepest part and reaches the 

 base of the appendage. The ijctiolus is composed of two 

 finely pointed pieces, which are closely approximated in the 

 living mite, and project in the middle line very slightly 

 beyond the margin of the appendage. There are five or six 

 pairs of very short hairs on the end of the body. 



Genital ^j/a^es- large, sinuate anteriorly, and gradually 

 narrowing towards the sides of the body, which they do not 

 overreach. J'J/jiineral jjlates remarkably long and narrow, 

 rather closely resembling those of ^4. sinuator, .AluUer. 



Palps stoutly built, with prominent distal angles to the 

 segments. The inner surface of the second segment seems 

 to be without a hair-pad, but carries a few stout unfeathered 

 bristles. Fourth segment with a long straight spine near the 

 inner distal corner and a widely forked tactile hair on the 

 apical margin. 



The legs do not present any unusual characters ; they are 

 rather stout, of moderate length, and the fourth segment of 

 the last pair is without a spur. 



Measwefnents. 



Length of body (including appendage). ... 1-28 niui. 



Breadth of body about ItK) nun. 



liieadtli i)f appendage at base about 870 /i. 



Length of palp about 430 /t. 



