596 LECTURE XXIII. 



Stone in Dumfnessliire : they indicate a liigh temperature of the 

 then existing climate of that northern latitude. 



I liave ah-eady alluded to the slightly undulating contour of the 

 margins of the septa in the Nautilus, which makes the surface next 

 the aperture of the shell convex at one part and concave at another. 

 In an extensive genus of extinct chambered shells called " Am- 

 monites," no example of which has been discovered in strata more 

 recent than the chalk, but which were exemplified under a rich di- 

 versity of forms before their final disappearance, the sinuosity of the 

 margins of the septa is much greater, and most of the surface next 

 the outlet is convex : the siphon perforates the septa at their centre 

 in extremely few species, and in the rest is situated at that margin 

 which is next the outer or ventral curve of the shell.* Certain 

 chambered shells thus characterised are straight, like the Ortliocera- 

 tites, but generally compressed, with their numerous septa joining the 

 outer shell by foliated dentations : they ai-e termed BacuUtes. In 

 the true Ammonites the shell is discoid and coiled upon itself as in 

 the Nautilus ; but it is strengthened by arched ribs and dome-shaped 

 elevations on the convex surface, and by the tortuous windings of tho 

 foliated margin of the transvei'se partitions. Separate casts of the 

 interior of the ohumbers are not unfrequently obtained, which have 

 become detached by the solution of the calcareous walls and septa of 

 the shell, or are held together by the interlocking of the dove-tailed 

 lobes of the raai'gins of the chambers. The last chamber of the Am- 

 monite was of large size, as in the Nautilus, and doubtless contained 

 all the soft parts of the animal save the small proportion which Avas 

 prolonged into the siphon. The aperture of the inhabited chamber 

 was closed by an operculum. Certain species, described by Mr. 

 Pratt f, from the Oxford clay, were characterised by the production 

 of a long and narrow process from each side of the mouth of the 

 shell, indicating a corresponding modification in the lobes of the 

 mantle, analogous to that which produces the auricular appendages of 

 the mouth of the shell in certain Argonauts. The same gentleman 

 has likewise recently shown me a small Ammonite, in which the 

 mouth of the shell is arched over transversely by a convex plate of 

 calcareous matter continued from the lateral margins of the outlet, 

 and dividing this into two apertures, one corresponding with that 

 above the hood of the Nautilus, which gives passage to the dorsal 

 fold of the mantle ; the other with that below the hood, whence issue 

 the tentacles, mouth, and funnel : such a modification, we may 

 presume, could not take place before the termination of the growth 



* The dissection of the Nautilus proved that what had been called the ventral 

 part of the aninioiiite was the dorsal. 



■j- Annals of Nat. Hist., N(jven»l)cr, 1841. 



