VERTEBRATA: REPTILIA. 537 



Fossil Cheloniida, Emydida, and Trionycidce occur, also, from 

 the Upper Oolites to the present day, the Eocene period being 

 peculiarly rich in their remains. In the Tertiary deposits of 

 India (Siwalik Hills) there occurs a gigantic fossil Tortoise 

 the Colossochelys Atlas which is believed to have been 

 eighteen to twenty feet in length, and to have possibly sur- 

 vived to within the human period. 



ORDER II. OPHIDIA. The second order of Reptiles is that 

 of the Ophidia, comprising the Snakes and Serpents, and dis- 

 tinguished by the following characters : 



The body is always more or less elongated, cylindrical, and 

 worm-like, and whilst possessing a covering of horny scales, is 

 always unprovided with a bony exoskeleton. The dorsal ver- 

 tebrce. are concave in front (procoelous), with rudimentary trans- 

 verse processes. There is never any stenium, nor pectoral arch, 

 nor fore-limbs, nor sacrum, and as a rule there are no traces 

 of hind-limbs. Rudimentary hind-limbs, however, are occasion- 

 ally present (e.g., in Python and Tortrix). There are always 

 numerous ribs. The two halves or rami of the lower jaw are 

 composed of several pieces, and the rami are united anteriorly by 

 ligaments and muscles only, and not by cartilage or suture. The 

 lower jaw further articulates with the skull by means of a 

 quadrate bone (fig. 288, a) which is always more or less 

 movable, and is in turn united with the squamous portion of 

 the temporal bone ("mastoid bone"), which is also movable, 

 and is not firmly united with the skull. The superior maxillae 

 are united with the praemaxillae by ligaments and muscles only, 

 and the palatine arches are movable and armed with pointed 

 recurved teeth. Hooked conical teeth are always present, but 

 they are never lodged in distinct sockets or alveoli. Functionally 

 they are capable of performing nothing more than merely hold- 

 ing the prey fast, and the Snakes are provided with no genuine 

 masticatory apparatus. The heart has three chambers, two 

 auricles and a ventricle, the latter imperfectly divided into two 

 cavities by an incomplete septum. The lungs and other paired 

 organs are mostly not bilaterally symmetrical, one of each pair 

 being either rudimentary or absent. There is no urinary bladder, 

 and the cloacal aperture is transverse. 



Of these characters of the snakes, the most obvious and 

 striking are to be found in the nature of the organs of locomo- 

 tion. The front limbs, with the scapular arch and sternum, are 

 invariably altogether absent ; and the hind-limbs, if not wholly 

 wanting, are never represented by more than an imperfectly- 

 developed series of bones concealed within the muscles on 

 each side of the anal aperture, and never exhibiting any out- 



