RATTULID^E. 63 



a single vortex. Behind the head is a strong transverse fold, seen in retraction, but 

 obliterated in extension ; close to which projects horizontally backward a long antenna. 

 The whole structure bears a very close resemblance to that of M. carinata, from which, 

 however, it is distinguishable at a glance. The mastax and trophi are on the same 

 pattern ; but the right malleus is even still further reduced, only a slight vestige of it 

 remaining. The dorsal ridge is evident but very low, with an outline regularly and ele- 

 gantly curved. The foot is small and short ; the toe nearly straight, long, slender, 

 acute, closely embraced at its very base by several very short sub-styles. A copious 

 secretion of mucus is often seen running down like a cord, from the base, whose viscosity 

 is attested by the force with which the tip is moored to the glass. 



This very elegant and sprightly animal is well named, for its resemblance to a rat is 

 at once manifest, both in form and movement. It moves nimbly about among the 

 vegetation, now nibbling, now turning short, now scudding hither and thither by little 

 starts, whisking its long tail (toe) about in all directions. It swims gracefully and 

 rapidly, revolving often on its axis. The periodic evacuations of its small contractile 

 vesicle are thirteen in a minute. The species is often found in company of the finer De- 

 smidece, and from the alimentary canal being commonly distended with matter of a rich 

 golden-brown hue, I conjecture that some of these may form its ordinary food. In the 

 discharge of fseces, I have noticed such a quick closing contraction of the rectum at the 

 point where the intestine merges into it (yet without constriction of the whole tube) as 

 suggests a sphincter there : and the distinction between the coloured contents of the 

 intestine and the perfect clearness of the rectum is well defined. P.H.G.] 



Length. Of body and head, -j-^-g- inch ; of toe, ^ s inch ; total, ^. Habitat. Pools, 

 widely dispersed, not uncommon (P.H.G.). 



M. BICORNIS, Ehrenberg. 



(PI. XX. fig. 5.) 

 Monocerca bicornis . . . Ehrenberg, Die In/us. 1838, p. 423, Taf. xlviii. fig. 8. 



[SP. CH. Body fusiform-ovate, zvith long thick head armed luith two projecting 

 unequal spines; lorica not ridged; toe tioo-thirds as long as body-and-head, with a 

 bulbous base, and no sub-styles. 



The integument is truly a lorica, though more flexible than usual. It is truncate 

 at the neck, whence a thick cylindrical head protrudes, the anterior half of which can 

 contract by bringing the sides together in strong puckers. With much resemblance to 

 both rattus and carinata, there is a marked difference in aspect, from the greater 

 development of this head, and from the unequal spines which project over it ; of which 

 the left is medial, much the longer, and decurved. The absence, too, of any dorsal ridge 

 is noteworthy. The toe is slightly swollen at its base, but I cannot detect any sub- 

 styles, though Ehrenberg speaks of them ; it is slightly recurved. The right malleus 

 has here quite disappeared. The brain is of unusual length, even descending below the 

 long mastax, and the eye, of moderate size and a pale-red hue, is seated nqar its middle. 

 There are small gastric glands at the base of the stomach, and tivo similar vesicles 

 attached to the rectal end of the intestine. The contractile vesicle's periods are three 

 in a minute. In other points there seems little to distinguish the species from its 

 fellows. There are, however, two antennae, also unequal, which project, side by side, 

 beneath the chief frontal spine. I have seen an egg matured in the ovary, remarkable 

 for its small size : perhaps male. (Of. Monoc. valga, Ehr.) 



In ponds and lakes around London, I met with this species and the preceding, six- 

 aiid-thirty years ago ; I have occasionally found both since, the present the rarer. Yet 

 I have had this multiply in a phial ; so numerous and so large, as to be visible to the 

 naked eye. They glide slowly about, sometimes hanging to the glass, or playing around 



