198 SHOULDER-GIRDLE AND BREAST-BONE. 



pyriform, and have their bulbous end directed backwards and inwards. In the herbivorous 

 genera these parts do not chondrify (see Plate XIX, figs. 13, 14, which show those of 

 the Wombat, magnified one and two diameters; Plate XIX, fig. 11, shows these parts in 

 the Koala, nat. size; whilst those of the Macropidse are displayed in Plate XX). Very 

 young specimens show a thin film of hyaline cartilage investing the bulbous sternal end 

 of the clavicle (Plate XX, fig. 6, Halmaturus ; fig. 9, Bettongia ; and fig. 12, Petrol/ale, 

 all ten diameters). The end of the clavicle (el.) is seen through the thin prse-coracoid cap 

 (figs. 9 and 12, p. cr.), and this is again capped with a delicate mass of fibrous stroma (o. st.) 

 which lies on the bevelled edge of the narrow front end of the prae-sternum (p. st.). In a half- 

 grown Halmaturus (Plate XX, figs. 3 and 4, one and a half and three diameters), this omosternal 

 segment has become converted into a sterno-clavicular ligament (1.) ; in the Wombat a small 

 synovial cavity (Plate XIX, fig. 14, 1. sy.) may be seen ; but the ligament is not dense enough 

 to form a " meniscus." 



The Sternum of the Marsupials is typically Mammalian ; it is formed of a series of segments, 

 which are rather oblong, but have concave sides, and these are ossified ectosteally ; the prae- 

 sternum and the xiphisternum are considerably modified. The former (Plates XIX, XX) is broad 

 where it joins the first sternal ribs, from which it does not always become segmented, for instance. 

 in the Wombat (Plate XIX, fig. 14, p. st. s. r. 1.), and in the Halmaturus (Plate XX, figs. 3, 4, 

 5, p. st. s. r. 1.) : in the young Peirogale (Plate XX, fig. 1 1) a notch has appeared below, which 

 may be completed into a transverse cleft in the adult. The anterior or coracoid region of the 

 prse-sternum does not ossify ; it is very much compressed ; and what bony matter it does get comes 

 from a forward extension of the " pleurosteon," which forms the keystone to the first thoracic arch ; 

 whilst the anterior half of the prse-sternum belongs to the same somatome as the seventh cervical 

 vertebra. The sternal segments are perfect, as far as division of the cartilage is concerned, but 

 their connection is fibrous (Plate XIX, fig. 7), for no synovial cavity appears between them; 

 the first transverse cleft is the widest (see Plate XX, fig. 3), and is the first to appear. The 

 main " pleurosteon" is supplemented in old age by a pair of epiphyses at each end (Plate XX, 

 fig. 1) : these are apt to fuse together (Plate XIX, fig. 1) : five " pleurostea" to each sternal 

 segment is common to the Marsupials and to many of the Rodents. There are, as a rule, four 

 meso-sternal segments, the last of these in the Wombat (Plate XIX, fig. 13) belonging to two pairs 

 of ribs. The xiphisternum is like a cheese-knife, and the haft is ossified by ectostosis, whilst the 

 semicircular and often eared blade has one (Plate XIX, fig. 12 the Koala) endosteal patch, or 

 two, as in the Dasyure (Plate XX, fig. 2). In the young Halmaturus the broad part is elegantly 

 trilobate, as in the Cassowary (Plate XX, fig. 5, x) ; in Didelpliys (Plate I, fig. 1, x. s. f.), and 

 in the Koala (Plate XIX, fig. 12, x. s. f.), there is a primordial fenestra or " fontanelle." The 

 sternal ribs (s. r.) are never segmented from the vertebral portion (v. r.), but slowly develope bony 

 matter within, by endostosis (Plate XIX, figs. 1 and 8) ; there, is no differentiation of the 

 "costa intermedia;" and lastly, each sternal rib is articulated by a synovial joint to its own 

 sternal bar and to the one in front, just as the capitulum of the vertebral rib articulates with the 

 corresponding, and with the preceding centrum. 



