280 W. SAVILLE-KENT, CONTRIBUTIONS TO A 



shaped series. The seemingly anomalous form Pontarachna^ 

 figured in the year 1840 as forming a perfectly symmetrical 

 undivided circular genital area, will, it may be anticipated, on 

 reinvestigation be found to be composed of two closely apposed 

 semicircular tuberculate plates, and be thus shown nearlv to 

 approach what obtains in Pseudoatax. In but one type out of 

 the whole family group, i.e. the genus Eulais, do we find that the 

 genital area is in both sexes devoid of tuberculation, and, indeed, 

 genital plates as such are altogether suppressed. This excep- 

 tional instance is, however, of the highest interest, since in this- 

 genus we get a connecting link between the typical Hydrach- 

 nidae and the ordinary ambulatory mites which quite exception- 

 ally possess tuberculated genital areas, as shown by the non- 

 development of swimming-hairs upon the hindermost pair of legs 

 and in the non-participation of these appendages in the natatory 

 function. The males only of the genus Arrhenuriis are likewise 

 for the most part deficient in genital tuberculations, such circum- 

 stance being explicable by the fact that the generative organ, 

 in the male of this genus is specially modified and occupies a 

 distinct postero- terminal position. The import of the genital 

 plates or areas of the Hydrachnidae, with their more or less 

 numerically developed tubercles or acetabula, has so far been left 

 unexplained, but they may be interpreted as organs of adhesion^ 

 and also probably of touch, through the medium of which an 

 accurate approximation of the genital areas is brought about 

 during the pairing of the sexes, which takes place while the 

 animals are swimming in mid-water. 



Genus I. Pseudomarica Neuman. 



Body elongate depressed, cuticular surface soft with scattered 

 marginal setae ; all four pairs of locomotive limbs decussating from 

 the anterior extremity of the body, their epimera united together 

 so as to form a continuous plastron-like sternal shield ; genital 

 plates simple, elliptical, with three elongate ovate tubercles 

 situated close to the inner border of each respective plate, a stiff 

 seta developed from the posterior margin of either plate ; palpi 

 simple, subulate ; terminal joint of hindermost leg provided dis- 

 tally with one long and two short setae in place of a claw ; eye& 



