CETPTOCLEEDUS OXONIENSIS, 181 



account * of the development of this part of the skeleton, and a short description of 

 the growth-changes is appended. 



In the young shoulder-girdle (text-fig. 89, A) the scapula is already triradiate in 

 form, but the dorsal and, more particularly, the ventral rami are very imperfectly 

 developed. The two bones did not meet in the middle line, and there is yet no trace 

 of the extension backwards of the ventral rami to meet the anterior prolongations of 

 the coracoids. The clavicles articulate by their outer ends with the anterior border of 

 the scapulae and meet in a median symphysis, the structure at this stage being essentially 

 the same as in the primitive Sauropterygia. The ventral surface of the clavicles was 

 exposed, the scapulfe, or at least their ossified portion, not yet extending beneath them. 

 In the successively later stages the ventral rami of the scapulae grow inwards and 

 forwards beneath the clavicles and at the same time are gradually prolonged backwards 

 in the middle line towards the gradually developing anterior median prolongations of 

 the coracoids (text-fig. 89, B). Finally, the condition described above is attained, the 

 scapulae extending almost completely beneath the clavicles and meeting in a median 

 symphysis, which through the backward continuation of the bone becomes continuous 

 with the symphysis of the coracoids, the coraco-scapular foramina being completely 

 separated from one another (text-fig. 87). 



In the coracoids the chief growth-changes that take place are the prolongation 

 forwards in the middle line to join the scapulae, and the formation of the prominent 

 postero-lateral processes (PI. X. figs. 1 a, 1 b, p.e.p. ; text-figs. 87, 88). As has already 

 been pointed out, the prolongation inwards of the scapulae beneath the clavicular arch 

 causes the latter to become functionally unimportant or useless, and consequently in the 

 family Elasmosauridae it is extremely variable in form and is met with in all stages of 

 reduction. 



Fore Limh. — The fore paddle (text-figs. 90, 91 A) is chiefly remarkable for the great 

 expansion of the distal end of the humerus in the adult : in the young this charac- 

 teristic is not seen and only develops with advancing ossification (text-figs. 90, A-C). 

 The head of the humerus is strongly convex in full-grown individuals, and its roughened 

 surface shows that it was capped with cartilage. The tuberosity [tu.) is strongly 

 developed and forms a quadrate prominence on the postero-superior surface at the 

 upper end of the bone ; its upper cartilage-covered surface is continuous with that of 

 the head of the bone, or in individuals of advanced age separated from it by a 

 slight concavity. The anterior border of the tuberosity is continued down as a strong 

 ridge on the upper part of the shaft, while the posterior border forms a prominent angle 

 continuous below with the posterior border of the shaft. The shaft is oval in section 

 and increases gradually in width towards the distal expansion ; its postaxial border 



* "On the Development of the Shoulder-girdle of a Plesiosaur (Cryptocleidus cvoniensis, Phillips, sp.)," 

 Ann. Mag. Nat. Hist. [6] vol. xv. 1895, p. 333. 



