VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 81 



age of the shell, into 3, 4, or 5 chambers, by so many transverse, white, brittle, 

 semipellucid septa, each of which has a triradiated aperture, through which the 

 animal protrudes itself when in motion. As to the real nature of these septa, 

 Mr. Lightfoot does not pretend to guess at the intention of Nature in their for- 

 mation. If it should be said, that they only point out the different periods of 

 the shell's growth, and are nothing else but the limits or terminations of the 

 animal's periodical increase, Mr. Lightfoot will not dispute the opinion, but, 

 supposing it to be so, he asks whether it is not equally probable that the trans- 

 verse septa in all the nautili are nothing else? The inhabitant of this curious 

 shell is of the slug kind, but of the aquatic tribe, and has filiform tentacula, the 

 eyes being placed on the head of the animal, near their bases, and not at the 

 tip, as in the land kinds: the colour of the animal is a grey brown. Mr. Light- 

 foot gives the following specific character of this species, viz. Nautilus lacmtris. 

 N. testa spirali compressa umbilicata carinata, anfractibus tribus supra convexis 

 contiguis, apertura semiovata, septis triradiato-perforatis. Fresh-water nautilus. 

 Nautilus with spiral, compressed, umbilicated, carinated shell, with 3 wreaths 

 convex above and contiguous, semiovate aperture, and triradiate-perforated 

 septa. 



In Mr. Walker's publication on minute shells it is described under the name 

 of Helix lineaia; but the chambered internal structure was unknown to Mr. 

 Walker. 



Fig. 1, shell A, pi. 2, shows the shell of its natural size, with the umbilicated 

 side uppermost. Fig. 2, the same with the depressed side uppermost. Fig. 3, 

 the shell magnified, with the depressed side uppermost, and showing the living 

 animal. Fig. 5, the same magnified, with the umbilicated side uppermost. Fig. 

 4, the same in front, but cut away to the first septum. Fig. 8, the animal's 

 excrement. Fig. 6, 7, horizontal sections of the shell, in order to show the in- 

 ternal structure. 



The 2d shell, b, Mr. Lightfoot names Helix fontana or Fountain Helix, and 

 thus gives its character, viz. Helix testa compressa obtuse carinata, hinc umbili- 

 cata, anfractibus tribus utrinque convexis, apertura semiovata. Helix with com- 

 pressed, obtusely carinated shell, umbilicated on one side, with 3 wreaths convex 

 on both sides, and semiovate aperture. 



Fig. 1 shows the shell of the natural size, with the most convex side upper- 

 most. Fig. 2, the same with the umbilicated side uppermost. Fig. 3, the shell 

 magnified, with the most convex side uppermost. Fig. 4, the same with the 

 umbilicated side uppermost. 



This species vyas found in a spring of clear water among rotten leaves, its co- 

 lour is reddish brown or chesnut. 



VOL. XVI. M 



