1895.] HYDRACHNID FOUXD IX CORNWALL. 175 



The habitat of the creature is somewhat curious ; it was found in 

 a very small stream of fresh water, just where it comes tumbling 

 over the granite cliSs and runs down the sands into the sea. The 

 part where the Mites were found was the bottom, not the top, of 

 the clifE; the stream there is distinctly fresh water, but is only 

 three or four feet above high-water mark at the high spring-tides ; 

 so that when the wind is on shore and the sea at all rough con- 

 siderable quantities of salt-water must be carried into the stream, 

 and even at ordinary times a good deal of salt spray must reach it. 

 I searched in vain for the Aearid higher up the same stream, and I 

 have not hitherto succeeded in finding it in any of the other streams 

 in the same district. Although a Water-Mite, it is not found swim- 

 ming ; like other members of the genus it is adapted for crawling 

 only ; but I have not ever seen it crawling on the bottom or on the 

 water-plants, although of course it must do so. I have invariably 

 found it either in small chinks and splits in the rock, where it can 

 only be discovered by carefully chiselling away the rock in likely 

 places, or cHnging to the underside of large stones lying in deep 

 pools ; I thought from the latter position that the Acarids had been 

 carried down the stream, but, as before stated, I was not able to 

 find them higher up. 



The Mite is very conspicuous when its hiding-place is discovered ; 

 it is of a beautiful scarlet colour shaded and varied with orange, 

 and the soft cuticle is diversified by a number of porous plates of 

 clear yellow chitin sunk a little below the general level of the skin, 

 so as "to form shallow depressions. The legs are one of the most 

 striking features, as most joints are furnished with a radiating 

 whorl of large yellow spines tipped with scarlet, which give a veiy 

 brilliant appearance; the colours are difficult to preserve after 

 death. 



The Acarus appears to belong to the genus Thyas, the principal 

 characters of ^^-hich are as follows : — Hydrachnidae ^nth the eyes 

 placed at the side of the body (far apart), with two-jointed man- 

 dibles, without swimming-hairs on any of the legs, and with the 

 dorsal cuticle furnished with numerous separate chitinous plates. 

 It is by the last-named character that the genus is finally distin- 

 guished from Kramer's genus Aturus. 



I propose calling the new species Thyas petrophUus, from its 

 habits of life. 



Thtas petrophilus, u. sp. (Plate VII. figs. 1, 2.) 



mm. mm. 



Average length including rostrum about 1"00 1'35 



„ ,, of rostrum only about '10 "lo 



Greatest breadth about '65 "70 



„ thickness, dorso-ventrally, about '40 "50 



Length of legs, 1st pair about '38 "50 



„ 2nd,, „ ■... -50 -65 



„ 3rd „ „ -65 -85 



4th „ -85 1-10 



)) 51 TTUU ,, 



