768 DR R. BROOM ON 



the ' prseclavium.' From the development of the structure in Trichosurus and Pseudo- 

 chirus there seems little doubt that the element is a true sternal structure, apparently- 

 homologous with the anterior part of the sternum in Ornithorhynchus. Whether the 

 homology can be carried to the anterior element in the frog is less clear, but at present 

 Parker's name seems much the more appropriate. 



Conclusion. 



Id conclusion, the following may be regarded as a brief outline of the course of the 

 evolution of the marsupial shoulder girdle from that of the primitive terrestrial 

 vertebrates. 



In the Labyrinthodonts we have a shoulder girdle mainly cartilaginous, but sup- 

 ported by strong exoskeletal plates. Not improbably the endoskeletal apparatus 

 resembled that of Proteus, only the scapula being ossified. The exoskeleton consisted 

 of a large median plate and two lateral plates, corresponding to the interclavicle and 

 clavicles of higher forms, and a more delicate element — the cleithrum — adjoining each 

 lateral plate. When in Permian times the terrestrial reptiles came to be developed 

 from the amphibian forms, the chief changes that took place in the shoulder girdle were 

 that the endoskeletal elements became well ossified, and the exoskeletal elements became 

 more closely united with the other structures and less distinctly dermal bones. Such a 

 girdle is seen in Pareiasaurus, where the five exoskeletal elements of the Labyrinthodont 

 are still distinct elements, but where the scapula, coracoid, and precoracoid are well 

 ossified. 



In the further evolution in the mammalian line the cleithrum became lost through 

 becoming co-ossified with the scapula, forming its spine. Cynognathus probably 

 illustrates this stage. 



In the next stage a change, similar to that found in the monotremes, took place in 

 the ventral part of the girdle, the precoracoid becoming reduced. 



And finally, in connection with the many changes which gave to the mammalia 

 their typical characteristics, and of which the development of a soft flexible skin only 

 protected by hair probably was the most important, the coracoid became detached from 

 the sternum and much reduced in size to give greater freedom of movement to the 

 limbs. With this change the interclavicle would become lost to admit of the clavicles 

 becoming moveable. 



LIST OF LITERATURE REFERRED TO. 



(1) Broom, R., "On the Existence of a Sterno-coracoidal Articulation in a Foetal Marsupial," Journ. of 

 Anat. and Phys., vol. xxxi., 1897. 



(2) Gegenbaub, C, " Clavicula und Cleithrum," Morph. Jahrb., Bd. xxiii., 1895. 



(3) Seeley, H. G., " Researches on the Structure, Organisation, and Classification of the Fossil Reptllia. 

 vii. Further Observations on Pareiasaurus," Phil. Trans., 1892, vol. 183, B. 



