IV. rUJU'EBRATA : REPTILIA, SQUAMATA. 



597 



was an exoskeleton, some of the plates of which in the stegosaurs measured 

 a yard across. Among the characters of the group are the fixed quadrate, 

 jugal and postorbital arches, three to ten sacral vertebra, and ilium 

 elongate in front of and behind the acetabulum. Some of these forms 

 (Orthopoda) in pncuraatioity of bones, in having the pubic bones directed 

 backwards, and in the formation of an intratarsal joint, resembled the 

 birds, and have been regarded as the ancestors of that group. The Dino- 

 saurs were confined to mesozoio time. 



Order VII. Squamata (Lepidosauria, Plagiotremata). 



One of the characters which unite lizards and snakes and wliich 

 has given the name Plagiotremata is the transverse form of the 

 cloacal opening (fig. 625), behind which, in the male, are the 



ail/ 



ar 



Fig. 625. 



Fig, 626. 



Fig. 62.5.— Hinder trunk and hind limbs of a lizard. (From Ludwig-Leunis.) a, 

 cloaoal slit ; 6, femoral pores ; sta, anal shield. 



Fig. 626.— Skull of Ameiva ridgarw. an, annulare ; ar, articulare ; co, epipterygoid ; 

 cr, coronoid ; d, dentary : fr, frontal ; .7, jugal ; la, lachrymal ; ni, maxillary ; na, 

 nasal ; p, postorbital, above and behind it the parietal ; pf, prefrontal ; pr, pre- 

 maxilla; pt, pterygoid ; ij, quadrate ; qj, quadratojugal ; sq, squamosal ; tr, trans- 

 versum. 



paired copulatory organs, each lying in a sac from which tlioy can 

 be everted like tlie finger of a glove. The names Squamata and 

 Lepidosauria refer to the scaly condition of the skin. These 

 scales are horny structures and somewhat distinct from the bony 

 scales of fishes. The derma forms flattened papillaj which resemble 

 the scales of fishes in that in many species they contain bony 

 plates. These papillae determine the character of the epidermis. 

 Since the stratum corneum is espiecially thick on the top of the 

 papilla? and thinner between them, rhomboid and oval plates occur, 

 wliich either lie flush with each other (shields) or overlap like 

 shingles (scales). The rule is that the head is covered with regu- 

 larly arranged shields, each with its name, the trunk with scales 

 ill longitudinal, transverse, and oblique lines. Outside these is a 

 layer of cornified cells, the pseudocuticula, and outside of all an 

 inconspicuous true cuticle. Since all cornified cells are dead and 



