132 SHOULDER-GIRDLE AND BREAST-BONE. 



prae-sternum (p. st.) ; and fig. 7 shows that, like the " rostrum" of the Heron's Sternum, it is 

 laterally compressed. 



The three divisions of the Sternum are very distinct (fig. 8) ; the first of these is the prae- 

 sternum (p. st.) ; it is a many-sided plate, and the margins are not very straight. The anterior 

 margin (see fig. 9) is concave on both sides, and notched in the middle : this notch is the anterior 

 remnant of the original fissure between the sternal moieties. Antero-laterally the margins are 

 gently concave and deeply scooped (see fig. 7, p. st.), to receive the epicoracoids ; but the postero- 

 lateral margins are bowed out, and protrude in two places to receive the first two pairs of ribs : 

 the synovial surfaces for these are, as usual, cupped. There is a deep rounded notch on each 

 side, partly severing the "prse-" from the "meso- sternum" (m. st.) ; this latter part, at first very 

 narrow, widens gently, until it ends in the xiphisternal horns (x. st.) ; a conspicuous groove 

 indicates the original division of the meso-sternum into two halves. Pour pairs of sternal ribs 

 articulate with the meso-sternum by their rounded ends ; whilst a fifth pair, with pointed ends, 

 come nearly into contact with it. The meso-sternum is one-third longer than the prae-sternum ; 

 and it is exactly intermediate in length between the right and left xiphisternal horns ; the left of 

 these arcuate, pointed rods being one-fourth longer than the right (see fig. 8). These terminal 

 horns keep close behind the pointed semi-floating ribs, from which they have evidently been 

 segmented by a longitudinal cleft : if this cleft had not appeared the sternal horns would have 

 been one with the seventh thoracic ribs, just as the third thoracic ribs and the sternal horns form 

 one continuous bar in the Monitors (see Plate X). 



The sternal ribs are very thick at their upper third ; they then attenuate where they join the 

 " intermediate" part of the vertebral rib : this part, still wholly cartilaginous, like the sternal ribs, 

 is narrow below and then expands (fig. 7). The junction is here by a "synovial" joint, as 

 in the Cyclodonts, an ornithic character, and which also reappears in certain Mammalia : the 

 seventh joint is seen to be unfinished in front. There are two pairs of ribs joined to the manu- 

 brium (prae-sternum) ; these agree to the last cervical and first dorsal of the Tamandua ; it is 

 only by specialization that the proper typical number of manubrial ribs is attained ; these should 

 be only one pair, as we see constantly in the Mammalia. 



Behind the Sternum there are delicate bones looking like the abdominal ribs of 

 Chamceleo and Polychrus; they are very slender, and wholly ossified (see Plate XI, figs. 7 and 8, 

 a. sp.) ; they are only separated from the cutis vera by a little fibrous stroma, and are indeed 

 intermuscular septa ossified in their outer part ; these septa are the " inscriptiones tendinese 

 musculi recti." 



Cuvier ('Regne Animal') took this view of them; but Professor Owen ('Catal. Hunt. Mus.,' 

 vol. i, p. 158) speaks of them as "long and slender cartilages," and considers them to be part 

 and parcel of the costal girdles ; their real nature is that of subcutaneous splints ; they are, as 

 it were, the abdominal plates of the Chelonians broken up (see the various figures in Plate XII.) 



