VERTEBRATA. 299 



arms and tentacles of nearly equal length, furnished with a 

 double alternating series of horny hooks, from 20 to 40 pairs 

 on each arm ; mantle free all round ; fins large, medio-dorsal." 

 (Woodward.) Only one species is known, from the Oxford 

 Clay (Middle Oolites). High authorities, such as Owen and 

 D'Orbigny, question the validity of this genus, and regard it 

 as being founded upon specimens of Belemnites. 



d. Xiphoteuthis. Guard narrow and cylindrical, containing 

 a very long, deep-chambered, narrow phragmacone. Pro- 

 ostracum greatly developed (nearly a foot in length), very 

 narrow at its base, widening out anteriorly, and finally ter- 

 minating in a pointed apex. Only a single species is known, 

 from the Lias. 



CHAPTER XXVII. 

 SUB-KINGDOM VERTEBRATA. 



THE sub-kingdom Vertebrata may be shortly defined as includ- 

 ing animals in which the body is composed of a succession of 

 definite segments, arranged along a longitudinal axis ; the main 

 masses of the nervous system (brain and spinal cord} are situated 

 along the dorsal surface of the body, and are completely shut off 

 from the general body-cavity. The limbs are never more than 

 fottr in number, and are always turned away from that aspect of 

 the body upon which the main masses of the nervous system are 

 situated. In all, the nervous axis is primitively supported by a 

 cellular rod, which is termed the " notochord ;" but in most the 

 notochord is replaced in the adult by the bony axis known as the 

 " spine" or " vertebral column" 



The past existence of Vertebrate animals is chiefly recognised 

 by the preservation of their hard structures. These hard struc- 

 tures are of two kinds some belonging to the internal or true 

 skeleton (endoskeleton), others being of the nature of horny 

 or bony plates, scales, or appendages of various kinds, de- 

 veloped in the integument (exoskeleton). The nature of the 

 exoskeleton in the Vertebrates differs very much in different 

 cases, and it will be considered when treating of the separate 

 groups. It will be well, however, to give an extremely 

 general and brief view of the structure of the endoskeleton, 

 taking for this purpose a Mammal as a typical form. In this 

 way the student will be enabled readily to trace the modifica- 

 tions of the skeleton in the lower forms, and will without 



