TYLOTOTEITON. 851 



3rd caudal, the transverse process becomes flattened from before backwards, and 

 the bypapopbyses spring from it and from the centrum as a common base in a much 

 more perfect manner than occurs in the 1st caudal vertebra of Siren, as described 

 by Mivart. 



The hypaxial arch, figs. 15 to 18 m and n, consists of a broad flat plate, with two 

 diverging processes posteriorly. In the 4tli caudal vertebra, a prominent ridge is 

 developed on the anterior face of the plate, which is curved with its convexity 

 forwards ; and this modification becomes more and more intensified, until at last, 

 in the middle of the tail, fig. 16 m, the ridge is converted into a broad thin lamina, 

 with two divergent plates, which serve to embrace the ridges of the arch behind. 



The ribs.^—Th&YQ are sixteen ribs, thirteen in the dorsal, one in the sacral, and 

 two in the caudal region. Each rib is supported on a double-headed transverse 

 process, and is capable of a certain amount of mobility. As the ribs are traced back- 

 wards, the tubercular and capitular portions are less distinctly marked, as is also the 

 division of the transverse processes into dorsal and ventral sections. The ribs are 

 well developed in the five anterior vertebrae, and are moderately curved, but the suc- 

 ceeding ribs are nearly straight. The proximal ends of the ribs, figs. 19, 20, are 

 flattened from before backwards, and each consists of a capitular and tubercular 

 process, separated from each other by a narrow interval, except in those in which 

 these parts tend to become confluent. A considerable process (figs. 19, 20, &c., p) 

 is developed on the posterior margin of each of the first five ribs. All of these rib 

 processes, with the exception of the first, terminate in the knob-like glands of the 

 side, and the free extremities of the remaining ribs end in the same structures. The 

 first rib lies under the scapula, and has no gland opposite to it. • The sacral rib, 

 figs. 11 and 12 m, has such a strong resemblance to its transverse process that it 

 might be mistaken for such, and from its nature has of course no relations with the 

 lateral glands. 



Scapular arch. — The scapular arch lies over the 1st rib and on the anterior fourth 

 of the 2nd between the 2nd and 3rd vertebrse. All the constituent bones are com- 

 pletely united with one another. The coracoid element is a semicircular flattened 

 rather thick plate (fig. 41), separated from its fellow by the semi-osseous epicoracoid 

 flaps which overlap each other. The glenoid cavity is behind the line of the 

 scapula on the posterior and superior margin of the coracoid, and looks upwards, 

 backwards and outwards, and is triangular with its base upwards. It has two 

 protuberant walls ; but the upper and posterior margins are continuous with the 

 posterior margin of the coracoid, and scooped out into a deep wide notch, closed in 

 the recent state by a strong membrane. The lateral margins are separated from 

 each other anteriorly by a narrow notch for the reception of the articulating ridge 

 of the humerus. There is a nerve foramen in the usual position in front of the 

 glenoid cavity and a deep notch at the base of the anterior angle of the scapula 

 and coracoid. The scapular element is shghtly concave externally, and expanded 

 from below upwards terminates in a broad semi-osseous or cartilaginous supra- 

 scapula without any notch on its anterior angle. 



