MAMMALS. 223 



appearance of a prae- and meso-sternal cleft. In the lower types the Sternum is more like that 

 of a Rodent ; but, as in Man (fig. 14), the bony deposit is endosteal (fig. 17 a, Cercocebus). 



Lastly, the sternal ribs are slowly and feebly ossified by endostosis, and have no transverse 

 cleft to separate them from the vertebral ribs (figs. 9, 12, 17, 21, v. r., s. r.). In the flat-nosed 

 Monkeys (fig. 17, Cercocebtts cetldops, Kuhl, natural size; and fig. 21, Jacclms penicillatus, 

 Geoff., one-and-a-half diameter) the scapula is very oblique, as in the Bats ; the supra-scapula 

 (s. sc.) is a moderate cartilaginous angle ; the prse-scapular region (p. sc.) gives a supra-spinous 

 fossa one-third the size of the infra-spinous ; the meso-scapular spine (m. sc.) is large^ and termi- 

 nates in a large rounded acromion (ac.), which curves forwards, and in Cercocebus has an ossicle of 

 its own (figs. 17, 18, ac.). The coracoid (cr.) is a large, blunt, decurved hook; the coraco-scapular 

 notch is shallow ; the neck of the scapula broad ; the clavicle (cl.) strong and sigmoid, and 

 capped by a meso-scapular segment of cartilage (m. sc. s.) above, and a prse-coracoid segment 

 (p. cr.) below. In the Marmoset (Jacchus) the meso-scapular segment (m. sc. s.) breaks up into a 

 small piece, which joins the top of the clavicle ; and a larger (figs. 21, 22, three diameters), which 

 is ossified separately on the right side. The omosternum (o. st.) is always converted into fibro- 

 cartilage (fig. 20, Cercocebus, three diameters), between which and the prae-sternum (p. st.) there 

 is a synovial cavity : this is not so evident between the prse-coracoid and the omosternal plate. 

 In Cercocebus (Plate XVII, m. st.) there are five well-made meso-sternals, and then a " metosteon " 

 in the handle of the xiphoid process. In Jacchus there are only four well developed ; the fifth 

 being symmetrical, but arrested as small epiphysoid grains of bone. The fibrous tract cutting off 

 the prae- from the meso-stcrnum is not repeated ; the terminal plate of the xiphoid is most out- 

 spread in the Marmoset (fig. 21). 



The normal form of the manubrium is departed from in the Howling Monkeys ; in Mycetes 

 seniculus (Brit. Mus., see Plate XXVIII, fig. 19) the hinder half of the pree-sternum is normal ; 

 but in front its two large horns have not approached each other; are each ossified separately, the 

 ossifications answering to those in the costal processes of the Rhea (Plate XVII, fig. 7, pro.) : the 

 thoracic part of the manubrium does not carry the first ribs, but these are attached behind the 

 clavicles on the sides of the "horns." In Mycetes ursinus (?) (Mus. Coll. Surg., No. 4718 B; 

 see Plate XXVIII, fig. 20) the first ribs are attached to their proper keystone, and also to the 

 " horns," which are greatly cut away for the ribs close behind the clavicles. 



I have not worked out the development of these parts in Man from so early a stage as I 

 could have wished. 1 In the first stage observed by me (Plate XXX, figs. 1 3, an embryo two 

 inches four lines long, magnified four and eight diameters) the scapula has much of its persistent 

 form, and has its middle third occupied by an ectosteal bony sheath ; the supra-scapular region 

 (s. sc.) is large, wide, hooked in front, and premorse behind. 



The prae-scapular region (p. sc.) is as small as in the Whale and the Platypus; the meso- 

 scapular spine (m. sc.) is thick ; arises from near the front, at the middle of the plate, and is 

 only attached at its root ; all the rest is free as a long and stout acromion (ac.), which runs 

 downwards, and then turns forwards and inwards as a hook. The coracoid (cr.) is a blunt hook ; 



1 This has, however, been done by M. Gegenbaucr in his most valuable work, ' Untersuchungen 

 zur vergleichenden Anatomie der Wirbelthiere,' 1865 ; Zweites Heft. ; pp. 15 17. It is there shown 

 that a large-celled cartilaginous band is continued from the acromion to the prse-sternum ; here, how- 

 ever, the clavicle is not the ectosteal sheath to this band, but merely grafts itself upon the cartilage. 



