104 SHOULDER-GIRDLE AND BREAST-BONE. 



in their interior. In other typical Scaly Reptiles, when they were adults, I have been unable to 

 discover in the shield-like plate of the posterior sternal bone any such persistent cartilaginous streak 

 passing through it as in Varanus, but they appeared to be ossified through their whole length and 

 breadth. I have, indeed, in young specimens of Lacerta agilis, found that the ossification of the 

 plate in question commences in the interior of the two lateral halves, extending partly towards the 

 middle line, partly towards the surface. I have also in older specimens of a few other Scaly Reptiles 

 observed these plates still unossified near the surface, being covered with a very thin superficial layer 

 of cartilage. The bony substance of the shield-like plate, as well as the appanage of the posterior 

 sternal plate, which appendage likewise ossifies from within outwards, is much less strong and dense 

 than those of the anterior sternal plate [inter-clavicle], and exhibits a granular structure, whose 

 granules or compound bone-cells have a diamater of 0-0040 inch in diameter, and they resemble the 

 structure previously described as possessed by the posterior sternal plate [pra-sternum] of Pseudopus 

 Pallasii. 



IX. As already remarked, iu most of the typical Scaly Reptiles there is to be found at the 

 extremity of their posterior sternal bone an appendage directed backwards, which consists of one 

 [meso-ziphisternum~] or two [xiphisternal horns] bony pieces, that are movably connected with the chief 

 bone by fibrous tissue, and are rarely continuous with it. [The cleft is seldom quite through the cartilage.] 

 When two such pieces are present they appear like a couple of horns of symmetrical form and size, or 

 nearly so. For the most part these appear somewhat flattened on their upper and lower surfaces ; 

 occasionally, as in Draco viridis and Agama umbra, cylindrical in relation to their length they 

 have only a very moderate breadth and thickness. Posteriorly they terminate either between the 

 muscles of the abdominal wall or attach themselves to the extremities of two corresponding ribs, in 

 such a manner as to form an obtuse angle or a pair of arches. In the former case they narrow as they 

 pass outwards, and terminate in a point ; in the latter case they usually preserve the same breadth and 

 thickness to their extremities. 



Anteriorly, where they are attached to the plate of the posterior sternal bone, they lie in close 

 contiguity to one another when this plate has the form of a lozenge (Plate X, fig. 10), and are only 

 slightly supported by its posterior angle. On the other hand, they are separated by a considerable 

 interval when the above-mentioned plate is broad posteriorly, and presents the form of a regular 

 pentagon (Plate XI, fig. 2), their distance being, indeed, regulated by the length of this posterior side of 

 the pentagon in proportion to that of the other sides. In their course backwards they are either 

 parallel to one another or in the first instance diverge and then approximate, or, lastly, pursue 

 altogether a divergent course. In the process of development the horns of the Sternal bone are also 

 ossified from the centre towards the circumference, but a more or less thick layer of unossified cartilage 

 may frequently be found covering the surface. 



X. Sternal bones [free xiphisternals~\, with free extremities unattached to a pair of ribs, occur in 

 Draco viridis, Lophyrus giganteus, Lyriocephalus margaritaceus, Istiurus amboinensis, Basiliscus 

 mitratus, Moloch horridus, Phrynosoma Harlanii, Phrynocephalus caudivolvus, Agama mutabilis, Ag. 

 colonorum, Grammatophora barbata, Stettio vulgaris (Plate XI, figs. 1 and 2), Zonurus cordylm, 

 Uromastix spinipes. In all these Saurians the processes are bent outward, and are most distant from 

 one another at their extremities ; yet in the different species their curvature and relative position to one 

 another is so far different that in some, particularly in Draco viridis, Lophyrus giganteus, Lyrio- 

 cephalus margaritaceus, and Zonurus cordylus, they are for a very small extent in the first instance bent 

 outwards, but subsequently become widely divergent, whilst in other cases they are widely divergent 

 from the very commencement. The extremities are for the most part simply directed outwards ; but 

 in a few cases, especially in the species already adduced of the genera Draco, Lophyrus, Lyriocephalus, 

 Grammatophora, and Stellio (Plate XI, figs. 1 and 2), the ends are hook-shaped and are bent outwards 



