ANIMAL KINGDOM. 257 



guent au premier coup d' ceil par le double bouclier dans 

 lequel le corps est enferme, et qui ne laisse passer au 

 dehors que leur tete, leur cou, leur queue et lews quatre 

 pieds." Now, with the exception of the word quatre, 

 this description, which he considers distinctive, would ap- 

 ply admirably well to the Cephalopoda. The study of the 

 anatomy of a Chelonian reptile may serve to explain this. 

 Such an animal will be found to be inclosed between two 

 bony envelopes, one of which being more or less convex 

 constitutes the upper surface of the animal, and the other 

 being more or less flat constitutes the inferior surface. 

 The union of these round the sides of the animal forms a 

 thin edge or margin, analogous to that fleshy fin which 

 runs along the whole side of the Sepia officinalis. Nay, 

 in the soft tortoises, or the genus Trionyx of Geoffroy, the 

 margin of the animal ceases to be osseous and remains 

 constantly cartilaginous or coriaceous, so that the middle 

 only of the upper shell is osseous, in this exactly resem- 

 bling those Cephalopoda which have inserted in the middle 

 of their back a long oval, convex, horny, or calcareous 

 bone. We shall see hereafter that this is the first vestige 

 of a skeleton, and it may be proper therefore to describe 

 it more fully as it exists in the Cephalopoda. In the genus 

 Sepia this bone or shell is thick, oval, and composed of an 

 innumerable quantity of very thin calcareous laminae, pa- 

 rallel to each other and joined together by minute hollow 

 columns, which go perpendicularly from one to the other. 

 It may be asked, whether this apparently anomalous struc- 

 ture may not serve in some measure to explain the com- 

 position of bones in the Vertebrata\ At all events it can- 

 not be considered as a general characteristic of the Cepha- 

 lopoda; for in the genus Loligo this dorsal process is a 



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