468 MANUAL OF ZOOLOGY. 



ORDER IV. CROCODILIA. The last and highest order of 

 the living Reptilia is that of the Crocodilia, including the living 

 Crocodiles, Alligators, and Gavials, and characterised by the 

 following peculiarities : 



The body is covered with an outer epidermic exoskeleton 

 composed of horny scales, and an inner dermal exoskeleton 

 consisting of squared bony plates or scutes, which may be con- 

 fined to the dorsal surface alone, or may exist on the ventral 

 surface as well, and which are disposed on the back of the 

 neck into groups of different form and number in different 

 species. The bones of the skull and face are firmly united 

 together, and the two halves or rami of the lower jaw are united 

 in front by a suture. There is a single row of teeth, which are 

 implanted in distinct sockets, and hollowed at the base for the 

 germs of the new teeth, by which they are successively pushed 

 out and replaced during the life of the animal. The centra of 

 the dorsal vertebrae in all living Crocodilia are procoelous, or 

 concave in front, but in the extinct forms they may be either 

 amphiccelous (concave at both ends) or opisthocoelous (con- 

 cave behind). The vertebral ends of the anterior trunk-ribs 

 are bifurcate. There are two sacral vertebras. The cervical 

 vertebrae have small ribs (hence the difficulty experienced by 

 the animal in turning quickly) ; and there are generally false 

 abdominal ribs produced by the ossification of the tendinous 

 intersections of the recti muscles. There are no clavicles. 

 The heart consists of four completely distinct and separate 

 cavities, two auricles, and two ventricles, the ventricular sep- 

 tum as in no other Reptiles being complete. The right and 

 left aortae, however or, in other words, the pulmonary artery 

 and systemic aorta are connected together close to their origin 

 by a small aperture (foramen Panizzcz), so that the two sides 

 of the heart communicate with one another. The aperture of 

 the cloaca is longitudinal, and not transverse, as in the Lizards. 

 All the four limbs are present, the anterior ones being penta- 

 dactylous, the posterior tetraclactylous. All are oviparous. 



The chief points by which the Crocodiles are distinguished 

 from their near allies, the Lacertilians, are the possession of 

 a partial bony dermal exoskeleton in addition to the ordinary 

 epidermic covering of scales, the lodgment of the teeth in 

 distinct sockets, and the fact that the mixture of venous and 

 arterial blood, which is so characteristic of Reptiles, takes 

 place, not in the heart itself, but in its immediate neighbour- 

 hood, by a communication between the pulmonary artery and 

 aorta directly after their origin. 



The only other points about the Crocodiles which require 



