AxMMONlTES— ZOOLOGICAL^ ETC. 



107 



envelopes all the others, fA. semisulcatus, Thetys, pictura- 

 tus, Terverii, alpinus, Lm-gilliertianus, ^c.) the number 

 varies between eight and ten, the latter number being the 

 highest I am acquainted with amongst the Ammonites. 



Thus amongst the Ammonites, the number of lobes has 

 certainly a connection with the envelopement of the spires 

 by each other, as we find species whose whorls are only 

 contiguous, and not enveloped, have only two on each side ; 

 the species having the whorls half enveloped, vary between 

 the limits of five and nine lobes, on each side, whilst the 

 species whose whorls are entirely embracing, are provided 

 with eight or ten, attaining the maximum number at present 

 known. The reason of the increase in the number of lobes, 

 in proportion as the whorls become enlarged, is self-evident. 

 The animal would have a larger surface to cover with 

 ramifications in those Avhorls which were embracing, than 

 in those w^hich were not so ; and it would, therefore, be 

 requisite, either that the lobes and saddles should be 

 increased in size, or that there should be a larger number of 

 lobes. Nature has preferred the latter method. 



IRREGULAR SEPTA. 



It has always been thought, that the septa of Ammonites 

 were placed in symmetrical order, and corresponded perfectly 

 with the admirable regularity of the shell; that the dorsal 

 lobe for example, was invariably placed on the mesial line, 

 and that the others appeared at equal distances on each 

 side. This is not, however, the case, as I have found 

 exceptions to these rules in four instances. A, denarius, 

 splendens, Fitfoni, and Lyelli, in which the dorsal lobe, 

 instead of occupying the mesial line, is placed sometimes 

 to the right, and sometimes to the left of it, indifferently, 

 the middle lobe corresponding to the sides of the back. 



