28 Cephalopods, 



tural relations of its families along with the phases of their 

 progressive development in the order of time. The types of 

 the Ammonites and Nautili are the most ancient ; they even 

 appear very nearly contemporary in all their development, 

 and in this we may find a new proof of their value as zoolo- 

 gical groups ; however, they do not possess altogether the 

 eame importance. The family of the Ammonites, more nu- 

 merous and varied in the most ancient epochs, disappears 

 also sooner ; for it does not come further than the cretaceous 

 epoch. The researches of MM. De Buch and De Miinster 

 have made us too well acquainted with the order of succes- 

 sion of these fossils to render it necessary to refer to it here, 

 I shall merely remark that the genera, so curious and nu- 

 merous, which M. D'Orbigny has distinguished in the chalk 

 formations, where they appear in astonishing diversity, at 

 the very point where this family is about to become extinct, 

 furnish us with a very correct example, and certainly one 

 well worthy of fixing our attention, of the irregular, and, in 

 some degree, convulsive movements to which the ammon- 

 itigenic idea has been subjected in its expiring agony, with- 

 out reaching the tertiary epoch or the existing creation. 



The Sepiae, &c. form a third type of this class, and that 

 which occupies the highest rank in it ; its existence does not 

 appear to go beyond the lias, where the Belemnites, the Teu- 

 dopsis, and Celaenos have been the precursors of the Sepia?, 

 the Calmars, and the Onychoteuthes of our era. 



The department of the Articulata, like that of the mollus- 

 ca, and that of the radiata, contains only three classes, 

 namely, Crustacea, Insects, and Vermes. The other primor- 

 dial sections, which there has been an attempt to distinguish, 

 ought to be united under these three heads. Thus, the Cir- 

 ripedia can no longer be separated from the Crustacea, whose 

 organisation and mode of development they share. It is 

 likewise to the class of Crustacea that we must refer the 

 Lerneae, Rotiferas, &c. The Arachnida and Myriapoda are 

 true insects, or rather they are connected with winged insects 

 by intermediate types, so closely united that it is impossible 

 to separate them. We must not neglect, in these connec- 

 tions, the characters of the larvas and those of the species 



