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reptiles as a group are distinguished from the mammals in 

 many ways. Amongst other things they usually have a scaly 

 epidermic covering, but they never have hair ; they lay large, 



J9. 



FIG. 88. Series of tetraxon Sponge Spicules, showing how all may be 

 derived from the same fundamental type, highly magnified. (From 

 Dendy, Article " Sponges " in Encyclopaedia Britannica, Ed. XI.) 



heavily-yolked eggs and do not suckle their young ; they 

 have a cloaca into which the urinary and genital ducts, 

 as well as the alimentary canal, discharge ; they have a very 

 complex shoulder girdle (Fig. 89), consisting of a consider- 

 able number of more or less separate bones or cartilages, 



