THE ORDERS OF THE REPTILIA. 107 



tricle, the cavity of the latter being partially divided, by a 

 partition, into a right and a left portion. 



There is a urinary bladder, and the aperture of the cloaca is 

 transverse. The males have two eversible penes, one on each 

 side of the cloaca. 



The Lizards, the Blindworms, and the Chameleons, are 

 the best known forms of this order. 



3. The OPHIDTA. The snakes have no dermal, or osseous, 

 exoskeleton. The dorsal vertebrae are always procoalous, and 

 have a rudimentary transverse process, with which the simple 

 proximal ends of the ribs freely articulate. The front face of 

 each vertebra gives off a wedge-shaped process (zygosphene), 

 which fits into a corresponding pit (zygantrum) of the preceding 

 vertebra.* 



There is. never any trace of a sternum or of a pectoral arch, 

 of a fore limb, or of any sacrum ; but in some few snakes 

 (Typhlops, Python, Tortrix) there are rudimentary hind limbs. 



There is no interorbital septum in the skull, and its lateral 

 walls are completely osseous. The cartilaginous trabecuke 

 cranii remain distinct and persistent in the adult skull. 



The quadrate bone is always more or less movable, and is 

 generally united with the skull by the intermediation of the 

 squamosal. The rami of the mandible are united at the 

 symphysis only by ligament. The hyoidean apparatus is 

 exceedingly rudimentary. 



The teeth are never lodged in sockets, and are sometimes 

 grooved or canaliculated on their front faces. 



The heart is as in Lacertilia. The lungs and other paired 

 viscera are usually unsymmetrical. 



There is no urinary bladder. The copulatory organs are as 

 in Lacertilia. 



4. The CHELONIA. The Turtles and Tortoises always pos- 

 sess an osseous exoskeleton, which becomes intimately united 

 with parts of the endoskeleton to form a dorsal shield, the 

 carapace, and a ventral shield, the plastron. To this are 

 generally added epidermic horny plates, which form the so- 

 called " tortoise-shell." 



* Indications of this structure are found in some Lizards. 



