254 ORGANIC MIMAINS. 



the sipliimcle is more conspicuous in the nautilite than in the ammo- 

 nite; and whereas in the latter it commonly runs along the back, 

 particularly in the keeled species, in the former it is usually near the 

 middle of the whirl.* 



No. 2, PI, XllI, may be termed nautilus pompilius ; for it so 

 nearly corresponds with the recent nautilus of that name, that it may 

 be viewed as a variety of the same species. Sowerby's n. imperialis. 

 Tab. 1, almost coincides with our shell ; but the siphuncle in our 

 specimens is more central, as in the recent shell. There is, however, 

 considerable variation as to the disposal of the siphuncle, both in 

 recent and fossil specimens. This species, which is foimd in our alu- 

 minous strata, rarely displays the shell entire, the exterior being so 

 far gone, as to shew the edges of the septa; from the curved form of 

 which, it is denominated the lobster-tailed nautilus. It sometimes 

 is near a foot in diameter. — The n. scrobiculatus of Dillwyn, which is 

 very like n. pompilius, is a recent ammonite, shewing the interior 

 whirls. 



No, 1, PI. XIII, is the most common of our nautilites. It is a flat 

 umbilicated discoidal shell, often of a large size; the specimen figured 

 being sixteen inches broad. The aperture is somewhat ovate, the 

 back being rounded ; the sides towards the mouth are marked with 

 prominent ribs or undulations, which grow obsolete before arriving 

 at the back, and also gradually diminish in size as they recede from 



*It may be proper to observe, tliat what we call the back, viz. the exterior edge of the 

 disk, corresponds with what Mr. Sowerby names the front. We reject Sowerby's definitions 

 of the two genera, taken from ihe undulations in the margins of the septa ; because the characters 

 of a genus should be drawn from perfect specimens, not from imperfect; and the margins of the 

 septa cannot be seen, till the shell itself is destroyed or removed. Besides, great confusion 

 must arise from admitting Mr. Sowerby's distinction; and hence he himself has not strictly 

 adhered to it. We have an instance in nautilus truncatus, Tab. 123, in which the edges of the 

 septa are finely foliated, though the ribs or strise, which he seems to confound with them, are 

 straight. 



