750 ZOOLOGY SECT. 



the mesozoic period. The nautiloid Tetrabranchiata were 

 most abundant in the palaeozoic epoch, during which there 

 lived a great variety of forms of this group, some having the 

 shell straight (Orthoceras), or curved (Phragmoceras), or in a fiat 

 spiral with the turns not in contact, or in a helix, or a flat close 

 spiral (Nautilus, and others). The earliest representatives of the 

 Nautiloids are found in rocks of Cambrian age ; they are com- 

 paratively scarce in the mesozoic epoch and in the tertiary ; 

 and are represented at the present day only by the genus 

 Nautilus itself. The Ammonites are mainly mesozoic, the repre- 

 sentatives found in the earlier rocks (from the Upper Silurian 

 onwards) being few in number and simpler in structure than 

 the more typical later forms. The oldest fossil representatives 

 of undoubted Dibranchiata belong to the extinct order of the 

 Belemnites, which flourished in the mesozoic period from the 

 Trias to the Cretaceous, and survived in scanty number into the 

 Tertiary. Unlike the Tetrabranchiata, the Dibranchiata would 

 appear to have reached their maximum at the present day. 



The mutual relationship of the various groups of Cephalopoda 

 are indicated, as nearly as the information at our disposal will 

 allow, in the following diagram (Fig. 663). 



Decapoda 



\ Belemnifes 



Nauh'loids \ / ^Qcropoda 

 Ammonifes 



;. iy>3. Diagram to illustrate the relationships of the groups of Cephalopoda. 



GENERAL REMARKS ox THE MOLLUSCA. 



The Mollusca, like the Arthropoda, form an extremely well- 

 defined Phylum, none of the adult members of which approach 

 the lower groups of animals in any marked degree. There are, 

 however, clear indications of affinity with ' Worms," especially 

 in the frequent occurrence of a trochosphere stage in develop- 

 ment, in the presence of nephridia, and in the occurrence, in 

 Amphineura and some of the lower Gastropods, of a ladder-like 

 nervous system resembling that of some Turbellaria and of the 



