780 ZOOLOGY 



the largest of invertebrate animals. Like the other classes of 

 Mollusca they are most abundant in tropical and warm-temperate 

 seas. 



If the Ammonites are to be included among the Tetrabranchiata, 

 that sub-class was most abundantly represented during 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, the shell being straight 

 (Orthooeras), or curved (Phragmoceras), or in a flat 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 comparatively 

 scarce in the Mesozoic epoch and in the Tertiary, and are repre- 

 sented at the present day only by the genus Nautilus itself. The 

 Ammonites are mainly Mesozoic, the representatives 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 Tetra- 

 branchiata, the Dibranchiata would appear to have reached their 

 maximum at the present cfity. 



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. 713). 



Decapoda 



Belemnit-es 

 NatfMloids 



Ammonifes 



FIG. 713. Diagram to illustrate the relationships of the groups of Cephalopoda. 



GENERAL KEMARKS ON THE MOLLUSCA. 



. 



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

 defined phylum, none of the adult members of which approaches 

 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 trochophore stage in development, 

 and in the occurrence, in Amphineura and some of the lower Gastro- 



