252 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



In dealing with the Cephalopoda it is essential to take into account 

 the past history of the race, since so many, especially of the shell- 

 bearing forms, have long been extinct. 



The modern representatives of the class ' number close on 500 

 species, belonging to about 80 genera, of which total about half are 

 referable to the genera Polypus, Sepia, and Loligo, while only five 

 species, all belonging to the genus Nautilus, are possessed of an 

 external shell. 



The Nautiloidea,* which began in the Cambrian with seven straight- 

 shelled species representing four genera, attained their maximum in 

 the Silurian with about 230 species belonging to 20 different genera 

 and subgenera. Since that epoch they have steadily diminished in 

 numbers down to their minimum at the present day, while the 

 surviving genus. Nautilus, only made its first appearance in the Trias, 

 or, sensu stricto, the Tertiary. Nor did the vigorous offshoot of 

 Ammonoidea that started in the Devonian and attained to a countless 

 host of species, which from some monographs one might almost infer 

 were referable to an equal number of genera, succeed in keeping up 

 the number of testaceous Cephalopoda, for with the close of the 

 Cretaceous period the whole group died out after experimenting in 

 every type of shell-form in the effort to survive. 



Nothing is at present known of the embryonic development of 

 Nautilus, and we do not consequently know if the primitive, embryo 

 shell differs in any respect from the adult, but the fact that the earliest 

 Cephalopods had straight shells and that the line of development led 

 through curved to coiled forms is suggestive, and raises the speculation 

 whether the j^rimitive gastropod shell may not also have been straight, 

 and this phase in its development subsequently suppressed in its 

 embryonic history. 



Following up the scale of geological time, we meet with the first of 

 the decapods [Aulacoceras, belonging to the family Belemnoteuthidae) 

 in the Trias. It is interesting to note that, in the same series, the 

 earliest gastropod referable to the Tectibranchia, a species of liullinella, 

 is also recorded. So that we have a cephalopod with an internal shell 

 comparing in time with a gastropod of a group that only subsequently 

 in the chalk period achieved a partially internal shell [Philine). 



The Myopsida or next higher tribe of Cephalopoda began in the Lias 

 {Geotcuthis and Belotcuthis) ; while in the Cretaceous of Lebanon the 

 oldest known octopod, Pahcoctopus Nocboldi, makes its appearance just 

 as the Eelemnites and Ammonites disappear from the scene. 



So far as the shell is concerned, then, the Cephalopoda seem to have 

 been yet more eager than tlie Gastropoda to jettison the encumbrance, 

 and their predatory habits have obviously had much to do in hastening 

 this consummation. 



Cf. Hoyle : " Cat. Recent Cephalopoda " and "Supplement," Proc. Roy. Phys. Soc, 



Edinburgh, ix (1886), p. 207 ; xii (1897), p. 364. 

 Foord : " Cat. Fossil (Jeph. in Brit. Mus.," pt. ii, pp. xviii-xix. These and the 



following paragraphs have been kindly checked by Mr. G. C. Crick. 



