170 



REPTILIA 



CLASS III 



The anterior border of the coracoids is somewhat deeply excavated, the 

 posterior margin entire and rounded. Its lateral margin is developed into a 

 short, stout, and thickened process bearing two articular facettes, the foremost 

 for receiving the proximal end of the scapula, and the hinder for the head of 

 the humerus. The clavicles are slender curved bones, usually meeting in the 

 middle line at the anterior margin of the interclavicle. Their union may be 

 either by suture or fusion, or sometimes even by a joint. 



Humerus, radius, and ulna are all short stout bones (Fig. 271), and 

 relatively much reduced. The proximal end of the humerus is thickened, the 

 distal flattened, and provided with radial and ulnar 

 articular facettes, which are remarkable for being 

 concave. The remaining bones of the paddle, of 

 which the first two rows correspond to the carpus, and 

 the third to the metacarpus, are small and polygonal, 

 and usually arranged in a sort of mosaic. There are 

 commonly from three to five digits, but as some of 



IE" 



— IT 



cT 



id 





mc 



Fig. 270. 



Ichthyosaurus communis, Conyb. Lower Lias; England. Pectoral 

 arch and right fore limb, ventral aspect, V^ cl, Clavicle ; cor, Coracoid ; 

 /(, Humerus ; i, Intermedium; icl, Interclavicle (partly covered by 

 clavicles); 11, Radius; r, Radiale ; sc, Scapnla ; U, Ulna; u, Ulnare. 



Fig. 271. 



Ichthyosaurus tricissus, Quenst. 

 Upper Lias ; Boll, Wiirtemberg. 

 Left fore paddle, dorsal aspect. 

 '•', Distal carpals ; mc, Equiva- 

 lents of metacarpals. Other 

 letters as in Fig. 270. 



the phalangeal rows subdivide, the number of the latter is occasionally 

 increased to eight or nine. As many as 100 phalanges are sometimes present. 

 A deep incision is often observable on the radius and three succeeding bones 

 of the paddle, the significance of which is not understood. 



The pelvic arch is weakly developed, and freely suspended in the abdominal 

 muscles. Ilium, pubis, and ischium are all long and slender bones, and 

 apparently meet at the acetabulum. The femur is short and stout, but 

 relatively longer than the humerus, and the rest of the hind paddle practically 

 duplicates the structure of the anterior limb. The femur and humerus of this 

 order are unique in that, instead of having convex condyles for the articulation 

 of the epipodial bones, they present pre- and post-axial concavities for their 

 reception (Fig. 271). Impressions of the leathery integument are retained by 

 some specimens, as well as of the cartilaginous layer which surrounded the 

 phalanges and completed the outline of the paddles. Dermal armour was not 

 developed. 



