82 SHOULDER-GIRDLE AND BREAST-BONE. 



nified ten diameters, The supra-scapula (s. sc.), which was like a pointed knife in fig. 1, and 

 then had become obtusely angular by the retreating backwards of the apex, has now a much more 

 rounded upper surface; it will, however, have to undergo a great change of form before it has the 

 extended and arcuate upper margin of the adult (fig. 11). Besides the general extension of the 

 ectosteal sheath, there now crops out from beneath it the intercellular bone : this deposit may have 

 taken its cue from the applied bony layer ; still it has great independence of it, as a rule, in the 

 Anourans, and creeps to any distance beyond its boundary-line. A similar extension of both 

 outer and inner bone has taken place in the scapular region (sc.), and it is also seen that the 

 scapula is bifid below, although the chink (sc. f.) is very small in this species. Now is fairly 

 seen the relative feebleness of the prse-coracoid bar (p. cr.), and the increased energy in the life of 

 the coracoid : as for the ectosteal sheath of the former, this is its last appearance on the great 

 Vertebrate stage of life ; the prse-coracoid is dying out in the ascending scale of the Frog's specific 

 and individual history, and, as far as my observation extends, no creature with an amnion and an 

 allantois in its embryonic condition has this particular ectosteal sheath ; for when the cartila- 

 ginous bar itself is ossified from without, as in the Chelonians and Struthio camelus, it is by 

 a borrowed extension of the scapular sheath. 



In Plate V, fig. 10, the relation of the supra-scapula to the scapula is seen in a figure 

 magnified twenty-five diameters ; there is no hinge, and the endosteal deposit is seen to be 

 creeping beyond the outer layer. The " omosternum " (fig. 9, o. st.) is in a very instructive 

 condition, for its hinder end is very broad and crescentic, its tip is bifid a primordial mark ; 

 and an ectosteal sheath nearly enrings it, the ring being imperfect above, for the lower or outer 

 surface of these Shoulder-bones is always the most ossified, and the first to ossify. It is most 

 probable that in all perfect ectosteal sheaths there is a moment of time in which the bony matter 

 lies at some particular part of the circumference of a cartilaginous rod ; practically, many of these 

 rods may be said to be quite ensheathed from the first, because the growth of bony matter all 

 round is too rapid in many cases for the absolutely first stage to be seen. The Sternum (st.) has 

 progressed far towards its adult condition ; there is a soft prse-sternal bulb, a soft xiphisternal 

 symmetrical plate, and between them a short shaft-bone (meso-sternum and part of pra5-sternum 

 in one) : the xiphisternum, like the leaf of a Bauhinia, is one and yet double, as its bilobate 

 form shows. 



Stage 6, Rana F. I have now to describe the Shoulder-plates of the adult Frog ; and in 

 these structures we shall see the culmination of this morphological type. 



Plate V, fig. 11, shows these structures in an old male Frog (magnified four diameters), as 

 seen from above, and with the left supra-scapula removed. This region (s. sc.) is very large, 

 making some approach to its condition in Dactylethra (Plate VI, figs. 11,12). Its general 

 form is obliquely fan-shaped ; the narrowed lower part is concave at each side ; and the broad 

 upper part, with its rounded angles, forms two thirds of a transverse ellipse. The outer plate is 

 trifoliate, and covers one third of each surface, whilst all but the margin is affected by 

 endostosis. The whole region is convexo-concave, like a watch-glass, the outer side bulging 

 considerably. The scapula (sc.) has less of the foot-shape than is common in the Batrachia, the 

 prse-scapular (p. sc.) and scapular bars being nearly equal, and but little divided by the 

 " fenestra." 



The prse-coracoid (p. cr.) is adze-shaped, for the proximal part is bent at less than a right 



