MUEiENOSAUEUS. 107 



structure has been the subject of much controversy and can only be understood by 

 reference to the earlier members of the group, such as the Triassic Nothosaurs. In 

 these reptiles the shoulder-girdle consists of scapulae, enlarged coracoids, and a more 

 or less complete arch of bones forming an anterior transverse bar, of which the outer 

 ends are united to ventral processes of the scapulae. The constituent elements of 

 this bar are three in number, one median and two lateral, and it is concerning the 

 nature of these bones that discussion has arisen, some anatomists regarding them, or 

 at any rate the median one, as of sternal origin, others as clavicles and interclavicle and 

 therefore as membrane-bones. This latter interpretation has been strongly, and, in 

 my opinion, rightly, supported by the late Professor Seeley, and,asinmy former papers, 

 this view is hei'e adopted. The presence of this clavicular arch closely united at its 

 outer ends with the scapulee, seems to be correlated with the adoption of an aquatic life 

 and the consequent alteration in the direction of the pressure on the shoulder-girdle 

 through the glenoid cavity ; since under the new conditions instead of the fore limbs 

 helping to support the weight of the body, as they do in most terrestrial animals, they 

 now are mainly concerned in propelling it forwards through the water. The consequence 

 of this is, that the thrust of the head of the humerus on the glenoid cavity is directed 

 more or less forwards instead of mainly, upwards, and the presence of the transverse 

 clavicular bar attached to the scapulae is necessary to resist the forward and inward 

 thrust; and from the Nothosauridae through the Plesiosauridte to the most highly 

 specialised Elasmosauridae the modifications of the shoulder-girdle that have taken 

 place are in the direction of increased rigidity in that region. In the course of this 

 series of changes there is a tendency to the reduction of the clavicular arch accompanied 

 by, and consequent upon, the increasing size and importance of the ventral ramus of 

 the scapula. In the Triassic genus Nothosaurus the clavicular arch (text-fig. 61, B} 

 is a long curved bar composed of a pair of elongated clavicles, uniting at their median 

 ends with one another and with the small interclavicle, and externally uniting in suture 

 with a small ventral expansion of the scapulae. In most of the Liassic Plesiosaurs 

 (text-fig. 61, A) the ventral processes of the scapulae are increased in size, and in some 

 cases may be in contact with one another in the middle line. The clavicular arch, which 

 is overlapped below by the ventral processes of the scapulae, is modified in various ways : 

 as a rule, the interclavicle is large and extends back to the coracoids, so that the space 

 between the clavicular arch and the coracoids is divided into two separate openings ; 

 the clavicles may be much reduced. In the animal which has been called by 

 Lydekker* Thaumatosaurus arcuatus the condition is rather difi'erent, the clavicle 

 and interclavicle being both large and the latter not extending back to the coracoids. 

 but having a concave posterior border. In Munei.osaurus (text-figs. 62, 68), 

 Cryptocleidus (text-figs. 87-89), and other members of the Elasmosauridse the rigiditv 



* Catal. Foss. Eept. Brit. Mus. pt. ii. (1889) p. 163. 



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