40 



PROCEEDINdS OF THE NATIONAL MUSEUM. 



VOL. XXVIII. 



They are found on the mantle-folds or gills of the mussel, and feed on the 

 minute animals drawn in l)y the mussel. The late Doctor Haldeman 

 described a number of species under the generic name of Unionicola. 



The geiuxs yeuiiiaiinla is closeh' allied to Ata./\ l)ut peculiar on 

 account of the spirally ringed bristles on legs 1 and 11. 



The genus Ploim {Ciirr!2)ex) is also a large genus, and about 19 spe- 

 cies are noA\' known in North America. Thev occur chieHv in lakes 



Fig. 6ii.— .\t.\x f-p. 



Flci. (i7.— C<)X.\I, PL.\TES OF .\TAX. 



and ponds. The fourth joint of the palpus usually has two papilhv- 

 beneath, and the legs are well provided with swinuning hairs. The 

 genital aperture is much in front of the hind margin, and each side of 

 it are a great number of sucking disks, often (^f two sizes. 



Family HALACARID.E. 



This is a small family of marine mites. They have a leathery skin, 

 frequently granulate or striate, but commonh^ destitute of bristles. 

 Sometimes there are coriaceous plates or shields. The })ody usually 



shows the division into cephalothorax and 

 abdomen, both above and below. The ros- 

 trum is often quite prominent, sometimes as 

 large as in the Bdellidte. Thecephalothorax 

 usually has three eye-spots, one on the 

 middle in front. The palpi are three or 

 four-jointed, the last article sharp-pointed 

 at tip. The mandibles are rather promi- 

 nent, and end in a single straight, or recurved daw. The legs are mod- 

 erately long, rather widely separate at base, lateral or sub-lateral in 

 origin, and end in two claws. They bear a fcAV scattered bristles. The 

 genital opening is quite large and far back; the anus is small and at the 

 tip of abdomen. These mites have no tracheje, but do not appear to be 

 related to other atracheate Acarians, but rather to the Bdellida? and 

 Oribatidte. It is, perhaps, not a natural family, but derived from 

 several groups. The llalacarida> are found crawling slowh" over algse, 



i;s. — Claws of halacarus. 



