268 GEOGRAPHICAL AXD GEOLOGICAL DISTRIBUTION. 



Triassic genera Choristoceras, CocMoceras, Rhabdoceras, &c., which, 

 in their departure from the normal ammonitic type, resemble some 

 of these forms, but which differ in the simple character of the sutural 

 foliation. Considering the Ammonoidea to be the modified de- 

 scendants of the Nautiloidea, we have presented the somewhat 

 anomalous fact that while at the beginning of their existence in- 

 volution was the order of development, towards the end this develop- 

 ment was marked by a contrary evolution^ with an accompanying 

 approximation in outline to that of the primitive type-foiTns. No 

 facts with which we are at present acquainted permit us to state 

 what was the underlying principle involved in these reversed 

 changes. It can merely be said that, involution having once set 

 in, any broad departure from the type newly attained must almost 

 necessarily have been accomjianied by a certain amount of evolu- 

 tion. 



The persistence of type-structure among the Nautiloidea, and 

 the relation which the geographical distribution of the Ammonoidea 

 bear to supposed climatic zones, have already been discussed in 

 previous sections. Still more involved in doubt than that of the 

 Ammonoidea is the ancestry of the Belemnitidoe, which, as the earli- 

 est representatives of the two-gilled order of cuttle-fishes, first appear 

 in the Trias, and practically disappear with the close of the Meso- 

 zoic era, one speciec only, and that somewhat doubtful — Belem- 

 nites senescens, from Australia — being reported to pass beyond the 

 boundaries of the Cretaceous period.* That the group, however, 

 represents the ancestral line whence the recent Sepiophora (Sejaia) 

 have been derived there can be but little question, seeing how close 

 is the relationsliip between the determining jxirts — internal skeleton 

 — of the fossil and living species. In the Eocene genus Belosepia 

 the phragmocone is of a somewhat transitional character. The 

 modern pen-bearing cuttle-fishes or calamaries (Chondrophora) ap- 

 pear to have their direct ancestors in the various forms of Teuthidae, 

 whose remains, in a more or less perfect state of preservation, occur 

 in the Liassic and Oolitic deposits. 



Of the two other classes of Cephalophora, the Gasteropoda and 

 Pteropoda, only the former acquire any geological importance. 

 Beginning with a comparatively limited number of forms in the 



* The reference of this foiin to the Belemnitidre is considered more than 

 doubtful by Branco (" Zeitschrift d. deutach. gcol. Gesellschall," 1885J. 



