44 



THE GEOLOGIST. 



times a greatly compressed form. Superficially observed, 

 these differences have sometimes been made a motive for 

 the creation of species purely nominal^ as we may judge by 

 compressed or tumid individuals of A. interruptus, dena- 

 rius, Mantellii, varians, etc. In the examination of a great 

 number of specimens of each species, the distribution of 

 the lobes convinced me that no alteration in this important 

 character was caused by the greater or less degree of com- 

 pression, and that these individuals belonged to the same 

 species. The idea then struck me, from the analogy to what 

 we observe in Oliva and the other genera of the Gasteropoda, 

 where the sexes are separate, that these modifications might 

 depend on the difference of the animals inhabiting the shells; 

 that the most compressed specimens doubtless had belonged 

 to the males, whilst the tumid ones were those of the fe- 

 males. The animals of the latter sex being always larger 

 and shorter among the acetabuliform Cephalopods, on ac- 

 count of containing the eggs, I thought it probable that it 

 was the same amongst the Ammonites, of which the shell 

 had necessarily followed the volume of the sex which had 

 inhabited it. Once convinced of this fact it remained to as- 

 sure myself of the other external modifications which gene- 

 rally accompany the sexes, and their limits. I remarked, 

 for example, that the shells of the males at the same diame- 

 ter have almost always the ribs more numerous, and that the 

 inner tubercles approach much nearer the umbilicus, whilst, 

 among the females on the contrarj^, the ribs are wider apart, 

 and the tubercles further removed from the umbilicus and 

 more salient fA. interruptus, denarius, Mantellii, varians, 

 Rhotomagensis) , The difi'erence of the sexes also produces 

 some modifications in the convolutions in some species ; I 

 have observed that A. denarius, interriLptus, latidorsatus, 

 have the external whorl much larger in proportion to the 

 entire diameter in compressed individuals than in tumid 



