218 SHOULDER-GIRDLE AND BREAST-BONE. 



sternal ribs are all segmented from each other, and the latter become completely ossified, but by 

 endostosis originally ; there are five pairs of floating ribs, with a free inferior part, and then five 

 pairs entirely vertebral. Neither the lateral joint, nor that of the sternal rib with the Sternum, 

 nor that of the " capitulum " of the vertebral rib with the " centrum," are synovial. The junction 

 of the sternal rib with the Sternum (fig. 24, st. s. r. 2, eight diameters) is by the intervention of a 

 mass of fibrous tissue ; between the sternal and vertebral ribs (fig. 25, v. r. 3, i. r. 3, s. r. 3, 

 eight diameters ; and fig. 28, v. r. 2, i. r. 2, and s. r. 2, eight diameters ; and fig. 30, v. r. 3, 

 i. r. 3, and s. r. 3, sixty-four diameters) there is, in the midst of a mass of connective fibre, a 

 cartilaginous nucleus, which is partly split, and which answers to the intermediate rib. On the 

 " capitulum " of the vertebral rib (fig. 27, c. p.) there is a similar nucleus, and the tuberculum (t. b.) 

 is attached to the transverse process (t. p.) by fibrous tissue. The condition of the shortest form of 

 the Cetacean Sternum, that of the foetus of the Greenland Whale (Balana mysticetus), is well shown 

 in the invaluable work by Eschricht and Reinhardt (p. 118). The whole of this cartilaginous Sternal 

 plate is wedge-shaped ; its halves have completely united ; it has a blunt-pointed, free xiphoid, 

 which grows directly from the trilobate prae-sternum : as in the Newts, there is no meso-sternum. 

 The end of the first vertebral rib (b) which does join the Sternum, and that of the second (b') which 

 does not, is capped with fibro-cartilage ; then comes the fibro-cartilaginous mass (e), and then the 

 sternal rib (d) composed, as in the Dolphin, of hyaline cartilage. It is evident that the two 

 layers of fibro-cartilage (c. e.), seen in the embryo of Baleena answer to the split intermediate part 

 shown in the Dolphin (Plate XXIX, fig. 30, i. r. 3). In Balanoptera robusta (Lilljeborg op. cit., 

 p. 283, fig. 4) ; in B.rostrata (Flower, ' Proc. Zool. Soc., 1864/ p. 393, fig. 9) ; in Physalus anti- 

 quorum (ibid., fig. 7) ; in Sibbaldius ScJdegelii (ibid., fig. 8) and others, the Sternum is merely a 

 prse-sternum (manubrium with a free xiphoid rudiment) : this is extremely short in Sibbaldius, 

 In the Sperm Whale (see Mr. Flower's forthcoming paper in the ' Trans. Zool. Soc.' for this year) 

 there are three pairs of sternal bones 1 ; and thus this type approaches the Dolphins in the structure 

 of the Sternum. In the simple sterna there appeal's to be but one osseous centre, or perhaps a 

 pair of symmetrical bones in some cases ; but in the Sternum of a young Balaina australis, which 

 is deeply bilobate in front, I found an ectosteal centre, in f ront, in the crescentic emargination : 

 it had commenced at the selvedge. 



s 



Ordo "SIRENIA." 

 Example. Manatus Americanus, Cuv. 



The scapula of the Dugong and Manatee is very much like that of the Seal (Plate XXX, 

 fig. 8), but the processes are more developed, especially the acromion, in the Manatee. That of the 

 Dugong, however, is not unlike the same part in the embryo of the Horse, which has a scapula 

 very much less triangular than the Cow. This bone in the Sirenia is totally unlike what is 

 seen in the Cetacea ; having a goodly supra-spinous fossa nearly as large as the infra-spinous ; 

 whilst the acromion and coracoid have none of the parallelism seen in the latter. The resem- 

 blance of the scapula in one case to a fan, and in the other to a woodman's bill-hook, is seen in 



1 Mr. Flower has shown me that, as a rule, there are three pairs of sternal bones in the Sperm 

 Whale ; of these, the first and second pairs may unite together ; or the second with the third. There 

 is a large manubrial fontanelle ; and the primordial fissure is never quite obliterated. In an old skeleton 

 from Scotland (in the British Museum) the third segment is only partially ossified by an azygous bone. 



