132 Transactions of the Society. 



seemed to disappear ; and we did not find it again until March 

 1902. 



Viewed laterally, the ventral surface appears almost flat ; the 

 dorsal surface arched, more especially in the lumbar region, whence 

 it tapers somewhat rapidly to the foot, projecting a short flap 01 

 tail over the cloaca. 



Viewed dorsally, it shows a slight lateral compression. 



The sub-prone face has the lips of the buccal orifice projecting 

 almost as much as in D. Hoodii. 



The lorica is of the type normal to the genus, with the dorsal 

 and lateral clefts well marked. 



The eye is situated on the dorsal side of the posterior extremity 

 of the large brain-lobe. It consists of a saucer-shaped conglomera- 

 tion of red pigment, in front of which appears usually one (some- 

 times a pair) of clear spheres such as are seen in Proales spp. 

 (Petromyzon). 



The foot is small and flexible, and carries the usual bunch of 

 stiff setse, which are, however, somewhat short. 



The tail-like projection over the cloaca, so exaggerated in 

 D. sterea, is present in this species to such a very small degree 

 that it does not interfere with these seta?. 



The toes, which are nearly straight blades (slightly upcurved),. 

 tapering to a sharp point, are very often carried over the back, one 

 on each side of the dorsal cleft, as in the case of D. eva, cf. fig. 13a. 



The food consists of flocculent matter. 



The jaws have the incus short and stout, and clubbed at the 

 fulcrum. The manubria are thin and tapering, and not crutched. 



In habits this rotifer is a free and steady swimmer. 



Size. — Total length yly in. to ^ij in. (140-110 //,); toes alone 

 7 £o in. (34 /*) ; breadth ^- - in. (39 p) ; height ^ in. (43 l i). 



Habitat. — Knowsley Park, Lancashire. 



Uncommon, only found in small numbers. 



The cervical eye places this species nearest to D. Hoodii and 

 D. ventripes, from which, however, it may readily be distinguished 

 by the following points : — 



1. Its one or sometimes two lenses, if they are lenses. 



2. Its almost straight, slightly upcurved toes. 



3. Its slight lateral compression. 



From D. Hoodii it is further distinguished by its thick, short 

 incus ; and from D. ventripes by its unbent shape of body, and 

 terminal foot. 



We have named this pretty and interesting species " Derbyi," 

 in honour of the Eight Hon. the Earl of Derby, KG., as a slight 

 recognition of his courtesy and kindness in permitting us to enter 

 Knowsley Park, and to visit his ponds which we have found so 

 prolific in rotifera. 



Thus we have been enabled to collect very large numbers of 



