REPTILES. 109 



and the azygous piece the inter-clavicle. The former are strong, gently arcuate rods, loosely at- 

 tached to the front of the Shoulder-girdle, and reaching from the base of the supra-scapula to the 

 part where the prae-coracoid bends backwards to become the epicoracoid. They meet (see fig. 2. cl.) 

 in front of and a little above the inter-clavicle (i. cl.), to the transverse bars of which they are 

 strongly fastened. Fig. 2 shows that they lie in a plane outside the Shoulder-girdle ; and this 

 position is more clearly seen as it regards the azygous bone (i. cl.). This bone, the inter- 

 clavicle, is a miniature dagger, a bony " dagger of lath ;" for it is a very fibrous bone, although 

 strong enough withal. Above and below (figs. 2 and 3, i. cl.), it has an ornamental knob in front, 

 and from these knobs proceed a pair of gently curved bars, which look backwards, anchor-like. 

 The main, or longitudinal bar, reaches to twice the extent of the sub -trans verse bars ; it is narrow 

 at first, but its posterior half is like a surgeon's lancet, in outline ; it is concave above and 

 convex below. More than a third of this bar underlies the Sternum (see fig. 2). 



Most authors have seen the counterparts of the human clavicles in the symmetrical splints, 

 but the azygous piece has been misunderstood entirely ; these clavicles, however, are pure, and 

 not mixed up at their extremities with any endo-skeletal part, as in Man and many other Warm- 

 blooded Vertebrates. The azygous piece, my "inter-clavicle," is the " episternum " of most 

 authors; but it has nothing in common with the so-called "episternum" of the Frog my 

 " omosternum ;" nor with the cervical, projecting portion of the Mammalian manubrium. In 

 relation to the thorax generally, these bars or plates may be called the "prae-thoracic"- and 

 " ento-thoracic" derm-plates ; the latter shows its nature well in the five-rayed form, which will 

 remind the least observant anatomist of the plastron -plates of the Tortoise. This is the true 

 hoinology of these three splints ; they do' correspond to the three anterior plastron-bones of the 

 Chelonian. 



The Sternum of the Iguana (Plate IX, figs. 1 3, st.), in the main part, is lozenge-shaped ; it 

 is slightly concave above (fig. 3) and convex below (figs. 1 and 2); it is rather wanting in bilateral 

 symmetry. 



The antero-lateral margins are grooved for the coracoids ; the groove is rather deep (see fig. 6, 

 which shows this part of the Sternum in section, with its two endosteal layers of bone, 

 magnified twelve diameters), and the lower lip of this groove (fig. 3, 1. c. 1.) is jagged anteriorly. 



The postero-lateral margins have elevated synovial facets for four pairs of sternal ribs ; and a 

 line drawn across in front of the second pair would separate the manubrium from the meso- 

 sternum : there is no sign of separation into segments, save the concavities between the synovial 

 facets. At the junction of the prse-sternal and meso-sternal regions, there is along the mid-line 

 an oval " fontanelle," or "primordial fenestra:" the blade of the inter-clavicle does not nearly 

 reach this open space. The ossification of the main Sternal-plate has taken place by symmetrical 

 endosteal deposits, but these have united at the mid-line to a considerable degree ; the lower 

 layer to much the greatest extent (see fig. 2), the upper by an isthmus behind the fontanelle (fig. 3). 

 Besides the unossified cartilage at the mid-line, all the margin continues soft, even in old age. 

 Fig. 4 shows a section magnified twelve diameters, made transversely near the end of the Sternum; 

 it cuts through the upper isthmus, but finds a gap in the lower layer of bone. Between the 

 fourth pair of ribs there are two much thicker bars, which are not segmented off from the 

 Sternum, but only suffer constriction where they join it ; these are the xiphisternal horns. These 

 " horns " bifurcate at about the distance of nine lines from the Sternum, and these forks are seen 

 to be the fifth and sixth sternal ribs. This connation is a matter of arrested development, which, 



