358 PERCY SLADEN TRUST EXPEDITION 



33. Nothrus prismaticus, n. sp. (PL 18, fig. 17). 

 Length about "3 mm. 



A very small species, about the size of N. monodactylus. Cephalothorax long, 

 pointed. Pseudostigmata far apart ; pseudostigmatic organs ? Abdomen divided by- 

 straight ridges into areas in different planes, giving a prismatic effect. Sides parallel, two 

 longitudinal ridges dividing the greater part of the abdomen into three regions, the 

 median horizontal, the laterals sloping downwards ; the j^osterior declivity truncated and 

 divided by ridges into two diamond-shaped and one triangular surface. Fairly long 

 peripheral hairs, especially at the posterior end. Legs short and not very strong. 



One specimen from moss, Mahe. 



34. Nothrus scotti, n. sp. (PI. 18, fig. 19). 

 Length about '6 mm. 



Cephalothorax almost an equilateral triangle ; pseudostigmata far apart, pseudo- 

 stigmatic organs long, filiform ; rostral hairs short and clavate. Abdomen parallel-sided, 

 somewhat concave dorsally but with the median area rising again slightly and bearing 

 five pairs of spatulate hairs ; four pairs of similar hairs on either side. Posterior end 

 truncate and bearing two blunt protuberances each carrying a long outwardly curving 

 hair. The middle posterior region protrudes at a lower level and bears two short clavate 

 hairs. The whole surface has a roughened or shagreened appearance. 



Found in company with N. seychellensis in moss from Mt Sebert, and named after 

 Mr Hugh Scott. 



HOLOTHYRID^. 



With the exception of the Ixodoidea, or ticks, hardly any of the Acarina approach in 

 size the curious Gamasoid mites, for which Thorell in 1882 established the family 

 Holothyridse. Gervais in 1842 had described one of the forms under the generic name of 

 Holothyrus, " a new genus of Acarina of the family Oribates." Thorell perceived its 

 nearer relation to the Gamasidae, but agreed that it approached the Oribatidse in some 

 respects. He saw, however, sufficient grounds for regarding these creatures as con- 

 stituting a family apart. They are certainly Gamasoid in general character. 



Besides their comparatively immense size — they may attain a length of 7 mm. — the 

 Holothyridae possess several remarkable characteristics. Perhaps the most remarkable is 

 the presence in the buccal cavity of a radula, strictly comparable to that of a member of 

 the Glossophora. Another striking feature is the great size and the complicated structure 

 of the female genital opening. This is large in the Oribatidfe, but in the Holothyridae its 

 relative dimensions are very much larger and its structure quite difterent. Instead of two 

 folding doors meeting in the middle line there are four moveable pieces, the largest hinging 

 from the posterior end, two side-pieces opening outwards, and a shallow lid-like piece hinging 

 in front. When open, the apparatus presents a comparatively immense gap in the body 

 of the animal. In material from the Seychelles Thon * found three species of Holothyrus, 



* Zool. Jahrb. Bd. xxiii., 1906, p. 677. 



