176 



EEPTILIA 



CLASS III 



impressioii on the dorsal side of the centra. With the exception of atlas and axis, all 

 the cervicals bear short, double-headed ribs, attached directly to tlie centra. Tkere 

 are about twenty dorsal vertebrae with well-developed transverse processes and single- 

 headed ribs. The two sacral vertebrae are cliaracterised by baving somewhat sborter 

 transverse processes and broader ribs than tbe rest. There are between tbirty and 

 forty candals, most of which bear short, single-beaded ribs articulated with their centra, 



and chevron bones whose 



f^''Xf'^^^--<^f;%!\scr+s.t^<lj. right and left halves do not 



P^f Jv-^ \.0 \ / fuse below. The abdominal 



ribs are very robust, and 

 consist of a median and three 

 paired lateral series. 



In the pectoral arch (Fig. 

 279) the coracoids are char- 

 acterised by their large size, 

 being considerably longer 

 than broad, and joined in 

 a long median Symphysis. 

 There is no sternum, and as a 

 rule no separate interclavicle, 

 but the clavicular arch is 



Fig. 278. 



Plesiosaurus macrocephalus, Owen. Lower Lias ; Lyme Regis, 

 Dorsetshire. A, Lateral ; J5, Superior ; C, Palatal aspects of skull, Vß- 

 ag, Angular ; art, Articular ; bocc, Basioccipital ; bs, Basisphenoid ; 

 d, dentary ; eept, Transversa or ectopterygoid ; fr, Frontal ; ipt, Inter- 

 pterygoid vacuity ; j, Jugal ; mx, Maxilla ; nar, External narial openiug ; 

 orb, Orbit ;, pa, Parietal ; pas, Parasphenoid ; pin, Pineal foranien ; pl, 

 Palatine ; pinx, Premaxilla ; prf, Prefrontal ; pt, Pterygoid ; ptnar, Pos- 

 terior nares ; ptf, Postfrontal ; pto, Postorbital ; qj, Quadrato-jugal ; 

 qu, Quadrate ; s, Supratemporal vacuity ; sag, Surangular ; so, Sub- 

 orbital vacuity ; st, Supratemporal or prosquamosal ; sq, Squamosal ; 

 V, vomer (from Woodward, after Andrews). 



Fig. 279. 



Plesiosaurus laticeps, Owen. 

 Lower Lias ; Dorsetshire. Pectoral 

 arch, ventral aspect, ^/lo- c?, 

 Clavicle ; cor, Coracoid ; icl, Inter- 

 clavicle ; sc, Scapula. 



represented by a single, broad, bilaterally symmetrical bone to which the coracoids and 

 scapulae are firmly united. The latter are somewhat extended ventrally underneath 

 the clavicular arch, but do not meet in the median line. 



The humerus is robust and distally much expanded, but without epicondylar 

 foramen or groove. This bone and the femur are noteworthy as exhibiting large, 

 conical epiphyses, similar to those of frogs and certain Chelonians. The radius and 

 ulna are both a little longer than broad. Four elements are usually present in the 

 proximal, and three in the distal row of the carpus ; the five constricted metacarpals 

 are similar in form to the phalanges. The phalangeal formula is inconstant, but the 

 third and fourth digits are always the longest, sometimes comprising as many as nine 

 phalanges. The bind limb differs in no respect from the fore, except that it is slightly 



