THE ORDERS OF THE REPTILIA. 109 



The limbs are converted into paddles ; the bones, from the 

 humerus onwards, becoming broad and flattened, and losing their 

 mobility on one another. The phalanges become very numerous, 

 and marginal ossicles are added to the pre-axial and post-axial 

 edges of the limb ; but the number of the digits does not ex- 

 ceed five. There is a clavicular arch formed by an interclavicle 

 and two clavicles. The pelvis is not directly connected with 

 the vertebral column. 



The teeth are lodged in grooves of the premaxillae, maxillje, 

 and mandibles not in distinct sockets. Their fangs are deeply 

 folded. 



Species of the genus Ichthyosaurus abounded during the 

 mesozoic epoch. 



2. The PLESTOSAURTA. No exoskeleton is known. The 

 centra of the vertebra? are flat, or slightly concave at each end, 

 and the neural arches unite with the centra in the ordinary 

 way. The dorsal vertebras have long transverse processes, 

 undivided at their ends, and articulating with the equally 

 simple proximal ends of the ribs. No sternal ribs or sternum 

 are known, but there are well-developed false, or exoskeletal, 

 abdominal ribs. 



There is a sacrum composed of two vertebrae, and the cervical 

 region is often extremely long. 



The snout is produced, and the external nostrils placed far 

 back near the large orbits. 



There is no bony ring in the sclerotic. 



The limbs are paddle-like, but the bones retain the normal 

 form much more than in the case of the Ichthyosauria. There 

 are no marginal ossicles. A clavicular arch, formed of clavicles 

 and interclavicle, seems to have existed in some species, if not 

 in all. 



The scapula has a remarkable form, and sends out a pre- 

 glenoidal process, as in the Chelonia. 



The genera Plesiosaurus, Simosaurus, Nothosaurus, which 

 constitute this group, are mesozoic marine reptiles. 



3. The DICYNODONTIA. These strange reptiles (Dicynodon, 

 Oudenodon) are known only from strata of triassic (?) age which 

 occur in both Africa and India. They are not known to possess 



