72 SHOULDER-GIRDLE AND BREAST-BONE. 



form it is very much like what we have been describing, but on closer inspection it is full of 

 differences. 



In the first place it combines the Toad-character of no episternum with the Frog-character of 

 a shaft-bone in the true Sternum; then, instead of having " independent endostosis," this process is 

 altogether under the influence of the ectosteal sheaths ; so that the cartilage which appears 

 beyond these plates is entirely soft; this and the large size of the epicoracoids (e. cr.) bring us 

 back again to the Urodela. Nevertheless, the supra-scapula (s. sc.), prsc-coracoid (p. cr.), and 

 coracoid (cr.) are morphologically normal; the coracoid fenestra (cr. f.) is small and tear-shaped. 

 The sternum (st.) is a long flattened ectosteal bar, with a rounded prsesternal cartilage, anteriorly, 

 and a transversely oval xiphoid plate, equally soft, behind. 



Example 6. Bufo vulgaris, Lauren ti. 



My dissections of the Shoulder-plates of the common Toad are figured in Plate V, figs. 

 15 17, magnified seven and twenty-five diameters; and in Plate VII, fig. 4, magnified four 

 diameters : the latter is from a half-grown specimen, and the former from one a few weeks 

 after its metamorphosis. The structures of this smallest individual are spread out to show the 

 exact relative size of the segments. There is less of the bony sheath of the supra-scapula (s. sc.) 

 on the upper surface (fig. 15) than on the lower (fig. 16) ; its jagged edges and the underlying, 

 independent, endosteal plate are well shown. The scapula (sc.) is completely ensheathed with 

 bone ; it is a rather solid bar, and the base is perforated by the scapular fenestra (sc. f.), which 

 is seen above, but below opens into the glenoid cavity (gl.). Over this cavity there is (fig. 

 15, gl.) an independent patch of intercellular bone. Like the supra-scapular outer layer, that 

 of the prse-coracoid (p. cr.) is more below than above ; it merely clamps the anterior margin of 

 the cartilage. The coracoid (cr.) is a complete ring of bone, but it has not reached the ends of 

 its own region ; the coracoid fenestra is large and oval, and the epicoracoid (e. cr.) still quite 

 soft, and overlaps its fellow, the left overlying the right. The most important thing to notice in 

 this stage is the coalescence of the anterior part of the right and left prse-coracoids (fig. 17 shows 

 this part, magnified twenty-five diameters, and in this figure are seen the proliferating cartilage-cells, 

 lying in a profusion of intercellular substance) ; this is an unmistakeable Plagiostomous character ; 

 all the more to be noted because of its recurring together with " independent superficial endostosis." 



The Sternum (st.) at this stage has almost acquired its permanent form ; it is somewhat 

 hourglass-shaped, slightly emarginate behind, and wholly soft. 



In the half-grown species (Plate VII, fig. 4) the supra-scapula (s. sc.) is broader at the 

 upper margin, and the anterior edge from being convex has become concave. The ectosteal layer 

 has become hatchet-shaped, the blade lying below, and the edge looking backwards ; whilst the 

 endosteal bone has, through the rapid growth of the cartilage, become divided into an upper and 

 a lower patch ; these patches do not conform to the outer bone ; the scapula (sc.), the coracoid (cr.), 

 and the prae-coracoid (p. cr.), have advanced both relatively and really ; and only the edge of 

 the hinder, broad part of the epicoracoid (e. cr.), is now soft, for the " endostosis " has been 

 active since the earlier stage. 



The Sternum (st.) has grown longer in the body and broader in the xiphoid process (x. st.) ; 

 the anterior part of the prae-sternal region, and nearly all the xiphi-sternum is still soft ; the 

 rest is ossified by endostosis only ; there is, however, only one patch of this granular bone. 



