Iviii Introduction. 



The number of the phalanges increases in the digits of both extre- 

 mities from the innermost digit outwards, and reaches its maximum 

 in the fourth digit. In the foot of the CrocodiUna this digit has 

 no claw on its terminal phalanx, and the fifth digit is altogether 

 lost. 



Teeth are always present; except in Chelonia, where a horny 

 sheath covers the jaws, as in the bills of Birds. The teeth are 

 limited in CrocodiUna to the homologues of the bones which carry 

 teeth in Mammalia ; but in many Smiria they are carried upon the 

 pterygoid also ; and in Ophidia upon both palatine and pterygoid 

 bones, in addition to the mandibular, maxillary, and premaxillary 

 bones. Teeth are provided with sockets in Crocodili7ia, but in no 

 other existing Reptiles ; they are reproduced, as shed, during the 

 whole period of the life of these animals. The tongue may be 

 either spatula-shaped and immobile, as in Chelonia and CrocodiUna, 

 and some Sauria, or bifid, elongated, and protrusible, as in other 

 E-eptiles. The wide oesophagus, and the muscular stomach, are 

 ordinarily not unlike those of Birds ; and this resemblance is made 

 more striking in the CrocodiUna, which, in addition to the muscular 

 gizzard^ have a special porUo pylorica, such as is developed in many 

 grallatorial and natatorial Birds. But the digestive tract of 

 Reptiles, which are with few exceptions of carnivorous habits, 

 exhibits, in correlation with this uniformity of diet, fewer varia- 

 tions of arrangement than that of Birds. Labial and lingual 

 salivary glands are occasionally present. Of the former of these, 

 the poison-gland of OpJiidia is a modification. The liver and 

 pancreas have, as in Birds, two or more excretory ducts ; the latter 

 of the two glands is ordinarily perforated by the hepato -enteric 

 ducts ; a gall-bladder is always present, but is sometimes developed 

 upon the biliary duct at a distance from the liver. This gland is 

 unilobed in Sqiiamata, bilobed in Loricata. 



In the Squamate Reptiles and Chelonia, in which the heart has 

 not four distinct and separate cavities, tHe venous blood returned 

 from the system to the larger right auricle, is kept more or less 

 completely apart from the arterial, returned from the lungs or lung 

 to the left by the non-isochronism of the action of the anterior and 

 posterior parts of the ventricular cavity. The anterior or inferior 

 portion of the ventricular cavity is filled from the right auricle, 

 and empties itself into the pulmonary artery, and partly into the 



