SKELETON OF LIZARD. 623 



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

 under nervous 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 many cases, e.g. in some of the skinks, in Anguis, Heloderma, 

 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, where they are amphicoelous. 



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

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

 brae 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 premaxilfce 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- 

 temporal bones. The orbit is roofed by a series of small bones, of 

 which the anterior and posterior are respectively known as pre- and post- 

 frontal. 



On the floor of the adult skull there is a large basal bone, composed 

 of fused occipital and sphenoidal elements, and continued forward as a 

 slender bar (parasphenoid). This bone gives off two stout processes, 

 the basipterygoid processes, which articulate with the pterygoids. Each 

 pterygoid is connected posteriorly with the quadrate bone of the corre- 

 sponding side, and anteriorly with the palatine. From the union of 

 pterygoid and palatine, a stout os transversum or transpalatine extends 

 outwards to the maxilla. In front of the palatines lie the small vomers, 

 which, in their turn, articulate with the premaxilla and maxilla, both of 



41 



