406 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



and backwards respectively, as if to protect the 

 organ of sight. In a somewhat similar manner 

 the tentacular arms of the Cuttlefish whilst living 

 are maintained in a marked and definite attitude, 

 as may be observed in any aquarium." * 



We may conclude that the animals of the fossil 

 Nautilidoe, whether they had a straight shell like 

 Orthoceras, or an involute shell like Nautilus, were 

 not unlike in structure and habits to the living 

 species. They kept to the bottom of the sea, and 

 were able to propel themselves for some distance, 

 assisted by the buoyancy of the air-chambers. The 

 increase in the size of the chambers in Nautilus is 

 much greater in proportion to those in Orthocey^as, 

 and from the small size of these in the latter, as 

 compared with the length of the shell, it is difficult 

 to form an idea of what may have been the size of 

 the body of the latter animal. One would fancy the 

 owner of a six-feet shell (and some are believed to 

 have been longer than that) would be a large and 

 powerful fellow, yet the diameter of his body- 

 chamber would not be much over ten inches, and 

 if he could retract himself into it, as reported 

 of Nautilus, he may not have been so very big 

 after all. 



* Narrative of the Cruise of the Challenger, Vol. I., part i., p. 490 



Explanation of the Plate. 



Plate VI. 

 Orthoceras sp. ; longitudinal section § nat. size. 



a, a, air-chambers numbered in succession 1, 2, 3, &c. 



b, the same contracted. c, intervening space. 

 fZ, septum. s, siphuncle. 



In executing the drawing the aid of transmitted gas-light 

 was used. 



