284 VERTEBRATE SKELETON 



(fig. 307, C), reduced in the adult to two. The five carpalia are 

 separate. Both trochanters are close to the head of the femur. 

 The tarsus is reduced, one of the centraha taking part in forming 

 a tritibiale, and only three tarsalia are present. The most aberrant 

 fossils, the Thalattosauria, have the proximal appendicular bones 

 short and stout, in accord with the aquatic Hfe. The distal elements 

 are unknown. 



Squamata. — A few lizards are limbless, some have the limbs 

 greatly reduced. In normal genera the humerus has a roller head 

 between two well-developed tuberosities, and the dicondylic lower 

 end has an entepicondylar foramen. The ulna, stronger than the 

 radius, usually has an olecranon, and in many genera the lower end 

 of the radius is expanded. The carpal hinge is markedly .intracarpal. 

 The intermedium, distinct in the embryo, fuses with the centrale 

 (some have two centralia), the compound bone lying between radiale 

 and ulnare. The five carpalia are usually separate and the corre- 

 sponding phalanges are 2, 3, 4, 5, 3, but occasionally the fifth digit 

 has four or five phalanges. The digits of Chameleons are grouped 

 in two bundles, i and 5 being opposed to the others. The digits are 

 reduced to three in Seps, Chakides, etc., and reduction reaches its 

 extreme, short of entire loss, in some skinks. 



The femur is usually longer than the humerus and has only the 

 fibular trochanter, and the head is little prominent. Most species 

 have a patella. In the tarsus the fibulare enters a tritibiale, the 

 resulting 'astragalus' articulating with both crural bones, the intra- 

 tarsal hinge being distal to it. The tarsaha are reduced in number by 

 fusion, so that in some genera only 3 and 4 are distinct. The foot is 

 usually pentadactyle with a phalangeal formula 2, 3, 4, 5, 4. In 

 Pseudopus, Pygopus, etc., the foot is simple, there being no division 

 into digits, and in Ophisaunis, Aniella, Acontias, Anguis, etc., 

 all limbs are lacking. 



Pythomomorph appendages have many of the characters of other marine 

 reptiles (p. 282), the bones of the upper arm being short and broad, the radius 

 with its lower end expanded. The carpals vary from seven in Clidastes to two in 

 Mosasaurus and Tylosatirus. There were five digits with few phalanges in 

 Clidastes and Mosasaurus, but increased to eleven or twelve in Tylosatirus. 

 The hind limb, except in smaller size, is much like the other. The tarsal bones 

 are from one to three, and the four or five digits have a varying number of 

 phalanges. 



