383 THE ANIMAL KINGDOM. 



are of large size and form scutes placed on the neck, back and sometimes 

 on the ventral surface, or, as in the extinct Teleosaurus, covering the body 

 completely. In the Triassic Aetosaurus they overlap one another. Among 

 the Deinosauria the Stegosauria have large bony plates with spines. The 

 dermal skeleton of the thorax and abdomen is largely developed in the 

 Chelonia and forms a firm carapace and plastron within which the head 

 and neck, the tail and limbs can be withdrawn in many instances for 

 shelter. The carapace consists of a median series of eight ossifications 

 which, with the exception of the first and two last, unite each with the dorsal 

 spines of thoracic vertebra ; of a lateral series of costal plates united with 

 the ribs, and a marginal series sometimes absent (e.g. in Chelone midas) into 

 which the ends of the ribs are received. The plastron is imbedded in the 

 corium and consists of a median anterior entoplastron, and four pairs of 

 plates (hyo-, hypo-, xipho-plastra) placed one behind the other. These 

 plates cover the ventral surface more or less completely. In many Chelonia 

 the carapace and plastron unite firmly at their lateral edges, e. g. land 

 Tortoises. The abdominal walls are covered by a system of dermal bones 

 in the Plesiosauria and Ichtkyosauria, arranged in transverse rows ; each 

 row consists of a median bone pointed at both ends and at least three over- 

 lapping bones on either side. Most Reptilia possess claws. The integu- 

 ment is poor in glands. Scent-glands are present beneath the mandibles, 

 opening on the neck, or at the sides of the plastron in Chelonia. The 

 Ophidia have a pair of cloacal scent-glands, and the Crocodile similarly 

 placed gland follicles. Among Lacertilia^ Hatter ia has a pair of cloacal 

 glands, and in the majority there is a row of ventrally placed femoral 

 glands, the secretion of which appears to have a copulatory function. 



The conformation of the cranial bones varies much in the different 

 groups. The cartilage cranium is much reduced. The occipital region, 

 basisphenoid and alisphenoid are ossified ; but the presphenoid and orbito- 

 sphenoid are cartilaginous or imperfectly ossified and the ethmoidal region 

 is always unossified. There is an interorbital (ethmo-presphenoid) septum 

 present except in Ophidia. The pro-otic is usually free : the epi- and 

 opisth-otic fused with the supra- and ex-occipitals respectively : but in 

 Chelonia and Ichthyosauria the last-named remains free. The palato- 

 pterygoid region is well ossified and large in Chelonia and Crocodilia. The 

 series of bones developed in membrane is very complete. There are 

 frequently prae- and post-frontals present in front of and behind the orbits 

 respectively. The nasal bones are absent in post-cretaceous Chelonia. In 

 Lacertilia the quadrato-jugal is represented by ligament : it is ossified 

 however in Hatteria. The parasphenoid is very large in Ophidia^ and the 

 trabeculae cranii remain as two rods on either side of it in front of the basi- 

 sphenoid and behind the ethmoid. The vomers are double in Lacertilia 

 and Ophidia. The quadrate is immoveably united to the skull in Chelonia, 



