S7o REPTILES. 



by pores on the ventral surface of the thigh. Their secretion 

 is most obvious in the male at pairing time. The histological 

 composition of the skin is very similar to that of the frog's 

 skin. Pigment is deposited here also in two layers, of 

 which the outer is greenish, the inner black. It is of 

 special interest to notice that over the parietal foramen 

 (see Skull) the black pigment is absent, the green only 

 feebly represented; in this region, therefore, the skin is 

 almost transparent. 



Many lizards, such as the Chamaeleons, exhibit in a remarkable degree 

 the power of rapidly changing the colour of their skin. This is due to 

 the fact that the protoplasm of the pigment cells contracts or expands 

 under neivous control. The change of colour is sometimes advan- 

 tageously protective, but it seems often to be merely a reflex symptom 

 of the nervous condition of the animals. 



In a. few cases, e.g. some of the skinks, there are minute dermal 

 ossifications beneath the scales. 



Skeleton. — The backbone consists of a variable number 

 of vertebrae, and is divisible into cervical, dorsal, lumbar, 

 sacral, and caudal regions. Except the atlas and the last 

 caudal, all the vertebrae are proccelous, as in all living 

 Lacertilians except Geckos. 



The atlas consists of three separate pieces, its centrum ossifies as 

 usual as the odontoid process of the axis. There are two sacral verte- 

 bra with large expanded sacral ribs. To the ventral surfaces of many 

 of the caudal vertebrae Y-shaped " chevron " bones are attached. 

 Across the centre of the caudal vertebrae there extends a median 

 unossified zone ; it is in this region that separation takes place when a 

 startled lizard loses its tail. 



The ribs are numerous, but only five reach the sternum. 



The skull is well ossified, but in the region of the nares, 

 in the interorbital septum, etc., the primitive cartilaginous 

 brain-box persists. On the dorsal surface the bones exhibit 

 numerous impressions made by the epidermic scales, which 

 render it difficult to distinguish the true sutures of the 

 bones. As in Reptiles in general, the brain-case is small 

 in comparison with the skull, and is largely covered by 

 investing bones, between some of which are spaces or fossae. 



Two fused parietals with the rounded median "parietal foramen," 

 two frontals, and the two nasals, are the most important constituents of 

 the roof of the skull. Anteriorly, the premaxillse appear between the 

 nasals, while posteriorly the sickle-shaped squamosal is attached by a 

 suture to the parietal, and is overlapped by one of the two small supra- 



