1893.] ACAHtrs fottnd in cokn-wall. 263 



opportunity of studying the anatomy of two of these species. For 

 this purpose I required frequent fresh supplies of the creatures, but, 

 unfortunately, before I had completed my investigations, a two 

 days' storm of violent rain and wind came, and when I revisited 

 the stream I found almost all the alga washed away and not an 

 Acarus left in what remained. Under these circumstances I began 

 to search for other patches of the Cladopliora, and found a largish 

 quantity growing on the face of the cliff near the horn of the bay, 

 in a place somewhat difficult of access, nearer to the sea than the 

 other, but higher up the cliff ; a very thin thread of fresh water 

 trickled down the cliff here, keeping the alga moist, and the spray 

 of the sea would dash there in rough weather, and probably more 

 or less would be carried there whenever the wind was from the 

 sea, its most frequent direction. It was a warm corner facing 

 south. The weed was abundant and in good condition, having 

 been protected from the storm ; but, to my surprise, I did not 

 find in it a single specimen of either of the species which I 

 was looking for, but, on the other hand, I did find examples of 

 two other totally distinct species which I had not ever before 

 found in the Cladophora either at the stream in the middle of the 

 bay or elsewhere. The first was in great numbers and in all stages ; 

 it was one of the Halicaridae. 



More careful examination with a microscope disclosed that there 

 was also a second and much smaller species present in considerable 

 numbers ; it is, I believe, unknown, and the object of this paper is 

 to record it. The creatm-e, in spite of its minute size, has structural 

 peculiarities which seem to me to render it particularly interesting. 

 It belongs to the family Tyroglyphidae. I thought at first that I 

 had a new species of the genus Hericia, and that genus is certainly 

 its nearest ally ; but it is not possible to include it in that or any 

 other existing genus. The great and singular difference of the 

 two front pairs of tarsi and claws, the different position of the 

 anus, and the absence of the strong sexual dimorphism found in 

 Hericia, besides other points, distinguish it from that genus. 

 There is only one recorded species of Hericia, viz. H. robini 

 (Canestrini ex Eobin), a very singular creature of a flattened 

 diamond-shape (the male especially) which wades in the sap which 

 exudes from elm-trees where the bark has split. The present 

 species shares the flattened diamond-shape and the wading habits of 

 H. robini, although the habitat is so very different. 



The great peculiarity and interest of the present species consists 

 in the tarsi and claws. The Tyroglyphidae usually have rather 

 slender tarsi and from the actual end of each tarsus springs a 

 single claw either with or without a caruncle : where the caruncle is 

 present the claw is usually small ; where it is absent, as in the 

 genus Hericia, the claw is usually larger and more powerful. 

 The claw generally consists of hard colourless chitin, and is a 

 strongly curved hook with a very short, straight, hard, and solid 

 peduncle, at the proximal end of which a small ball-like swelling 

 serves to give attachment to tendons. Hook, peduncle, and ball 



