MAMMALS. 193 



the Chelonian and the Lizard. They are in nowise diminished in relative size ; and supposing 

 these dermal bones and the inner elements of the Shoulder to have been found fossil in the Lias 

 deposits, they would easily have been mistaken for those of a small Ichthyosaur. The symmetrical, 

 clavicles (Plate XVIII, figs. 4 6, cl.) are of the medium size ; they are simple, as in the Lizards, 

 that is, they have no superaddition of endoskeletal rudiments, as in the Bird and in all other Mammals. 

 They are dilated at the scapular end, and pointed below, where they lie upon the anterior margin 

 of the great azygous " interclavicle." This latter bone (i. cl.) is much larger than is usual even in 

 the Lizards ; it is in shape between a T and a Y ; and the arms, which are gently arcuate, nearly 

 reach to the acromion, whilst the clavicle overlaps it behind (see fig. 4). The arms of the inter- 

 clavicle pass insensibly into the body of the bone, which is very broad, and becomes much broader 

 and flatter behind, where it is somewhat four-lobed. 



A strong keel, rising in front, and running nearly to the end, makes this bone a close 

 imitation' of the " manubrium sterni," for which it has frequently been mistaken, whilst by 

 others it has been taken for the furcula : it answers, however, only to the angular plate of that 

 compound ornithic bone. The true manubrium, or praa-sternum, is as broad as the base 

 of the overlapping inter-clavicle ; it is composed of two parts, which are separated by an uncleft 

 band of cartilage. The anterior division of the pra3-sternum is only partially ossified by endostosis; 

 it is firmly interlocked between the inter-clavicle and the coracoids, but is free to some extent 

 above (fig. G, pr. o.). This ossification is an azygous representative of the symmetrical " pro- 

 ostea" of the Rhea, and it belongs to the seventh cervical vertebra. The transverse cartilaginous 

 tract (figs. 5 and 6) which separates the " pro-osteon" from the first " pleurosteon" (fig. 5) 

 is broad ; the latter bone belongs to the first costal arch, and is represented in Man by the whole 

 " manubrium." In the young Ornithorhynchus, this part (and perhaps also the succeeding 

 " pleurostea") is symmetrical (see Owen, ' Catal. Hunt. Mus.,' vol. i, p. 314, 315, No. 1G98). 

 There are three more " pleurostea ;" all of them, like the first, are keeled, and developed as 

 ectostoses ; unlike the "pro-osteon," the three " meso-sternal" bones (fig. 5) are much narrower 

 than the prae-sternal segments. 



There is no xiphisternum ; but this evidently arises from the fact, that the seventh sternal 

 rib has in it the substance of the xiphisternal horns, segmented altogether from the fourth 

 " pleurosteon," and without the longitudinal cleft, which should free it from the rib. The 

 first sternal rib (s. r.) is only partially ossified by endostosis, the rest are invested with ectosteal 

 sheaths, as in Birds ; but there is between each sternal and vertebral rib (v. r.) an intermediate 

 tract (i. r.) which is only partially ossified by endostosis ; this answers to the " costa intermedia " 

 of the Reptile. 1 A very clean cleft separates the sternal bones, and there is, I am satisfied, a 

 synovial cavity between them ; fig. 7 shows this in a diagrammatic form ; and fig. 9 the 

 cartilaginous surface of one of the ends (magnified three diameters) ; fig. 8 shows the cartilage- 

 cells (200 diameters). 



In Echidna histrix, the scapula (fig. 11, sc.) is higher, and less decurved; there is a distinct 

 supra-scapular endosteal bone (s. sc.) on the rounded posterior margin of the upper cartilage; 

 the meso-scapular process (acromion, m. sc.) is larger, and more forthstanding ; there is a small, 



' I am not acquainted with a single case in which any Mammal higher than the Monotremes 

 has ectosteal sternal ribs originally.; this is, therefore, a true ornithic character; for all Birds, except 

 the Penguin, show it. 

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