THE CEPHALOPODA 147 



pyrites : in which case no sutures are visible on the 

 surface. Only the finest ornament in cephalopods is 

 usually a thickening (capillae) or thinning (striae) of the 

 shell, but there are cases among ammonites where pro- 

 tuberant parts of the surface whether corrugations 

 (costae) tubercles or keel (carina) have their cavities 

 cut oft" from the general shell-cavity by a partition, so 

 that the cast fails to show these elevations or shows 

 them in a much smaller degree than the shell-surface. 

 Mr. Buckman terms these tubular structures septicosta 

 (found in dactyloids), septitubenles (in deroceratids), and 

 septicarina (in many oxycones). The partitions are 

 wanting in the greater part of the body-chamber, being 

 found in its posterior part and in the gas-chambers, so 

 that they are secreted by the hinder part of the mantle, 

 and like the chamber-septa are later in date than the 

 adjacent shell. These structures may be of considerable 

 stratigraphical value e.g., serving to distinguish Upper 

 Lias dactyloids from Upper Jurassic perisphinctids, 

 even in fragments. 



Among existing Cephalopoda two orders may be re-* 

 cognized, distinguished by the number of gills and various 

 other features. The order Tetrabranchiata (four-gilled) 

 includes the single genus Nautilus; all other living 

 forms belong to the Dibranchiata (two-gilled). It has 

 been usual to extend this classification to extinct forms, 

 but while many of these are so evidently allied to the 

 recent forms that they can safely be assigned to one or 

 other of these orders, there is one very important series 

 of fossils which diverges quite as much from the nauti- 

 loids in some respects as do the dibranchs. It is there- 



