SNAKES AND LIZAKDS. 18 



Order V.— PELYCOSAURIA (extinct). 



(Case 5.) 



Although at one time classed with the Theromorpha, the extinct 

 Permian Pelycosauria are now regarded as a distinct group, more 

 nearly allied to the Ehynchocephala, which they resemble in 

 possessing two temporal arches to the skull. The dentition gener- 

 ally approximates to that of the Theriodont Theromorphs. "Well- 

 known genera are Clepsy drops, Dimetrodon, Emholophorus, and 

 Naosaurus ; the three latter being characterised by the tall upright 

 spines of the trunk-vertebrte, which in some cases were equal in 

 length to the entire skeleton, and during life probably supported a 

 fin-like expansion of skin, as shown in the coloured sketch (46 f') 

 exhibited in the case. 



Order VI.— SQUAMATA. 



Snakes and Lizards. 

 (Cases 11-15 and 18-20.) 



Snakes and Lizards form at the present day the most numerous 

 representatives of the reptilian class. They are characterised by the 

 circumstance that the quadrate-bone (which forms the articulation 

 of the lower jaw) is more or less movably attached to the skull, as 

 well as by the presence of only one lateral bar (temporal arch) in the 

 latter, and by the teeth being welded to the jaws. The body is 

 usually covered with horny scales ; and the aperture of the vent 

 is transverse. 



The existing members of the group are divided into three sub- 

 orders : — 



L Ophidia, or Snakes. Characterised by the fibrous union of Cases 

 the right and left halves of the lower jaw, or mandible, the H-l^. 

 absence of functional limbs, of which (at most) only minute 

 vestiges remain, and the elongated form of the body. The 

 single eye-lid cannot be moved, and is transparent. 

 II. Lacertilia, or Lizards. In this group the right and left Cases 

 halves of the lower jaw are connected by a bony union. The 18-20. 

 great majority possess functional limbs, movable eyelids, and 

 horny scales ; but a considerable number have a more or 

 less completely snake-like form, with the reduction or loss 

 of one or both pairs of limbs ; and in some cases the eye-lids 



