102 SHOULDER-GIRDLE AND BREAST-BONE. 



to the comparative lengths of the two pieces, and is also influenced by the extent to which the anterior 

 piece projects forwards in relation to its length. Thus in Phrynosoma Harlanii, Moloch horridus, 

 Calotes pictus, Lyriocephalus marffaritaceus, and Opiums torquatus, it does not reach the second fourth ; 

 in Lacerla ocellata, Lac. agilis, Basiliscus mitratus, and Platydactylus guttatus, it reaches about to the 

 middle; in Anolis carolinensis and Seps chalcidica, far beyond the middle; and in Tejus Teguixin, 

 Ameiva vulgaris, and Uromaslix spinipes, it reaches nearly the end of the plate of bone formed by 

 the posterior [or true] sternal piece. 



That portion of the anterior sternal piece which lies under the posterior portion is received into a 

 corresponding fissure of the latter bone, which fissure, whilst it is for the most part only small, is yet some- 

 times, as in the Varanians, the Iguanians, in Tejus Teguixin, and Cyclodus nigroluteus, of consider- 

 able depth. It is usually fixed in this position by fibrous membrane, which can only be regarded as a 

 portion of the periosteum of the posterior sternal piece ; for while the furrow exists on this piece, the 

 periosteum, as may best be perceived on making a transverse section, splits into two layers of unequal 

 thickness, of which one invests the prolongation of the anterior sternal bone [inter-clavicle] on its under 

 side, whilst the other, much thinner, surrounds it on its upper surface. But in a few Scaly Reptiles, as in 

 Platydactylus gultatus and Phrynosoma Harlanii, the two sternal bones [Sternum and splint] are, after 

 complete development, no longer connected with one another by fibrous tissue, but have actually to a 

 greater or less extent coalesced. The substance of the anterior sternal bone [inter-clavicle'] is in the 

 typical as in the atypical species of Scaly Reptiles composed of strong and hard bone [hard very early, 

 being an ossification of fibrous tissue, " parostosis"] , the corpuscles of which are provided with numerous 

 canaliculi containing only a simple and proportionately small cavity, and lie close to one another. 

 They are not arranged in groups of any determinate form, but lie scattered irregularly in every part. 



VII. The upper or posterior and greater piece of the Sternum [true Sternum] in the typical 

 Scaly Reptiles has not only a proportionately greater extent, but also a more compound form than in 

 the atypical species, for it is provided at its extremity either with an azygous process [m,eso-xiphisternum] , 

 or with a pair of processes [xiphisternals^, which are connected to it by a fine fibrous tissue, and very 

 rarely, as in Polychrus marmoratus, coalesce with it [they are seldom quite segmented off, and when 

 continuous are so primordially] . We may, therefore, differentiate two portions in this piece, an anterior 

 and a posterior, of which the former constitutes the most essential part of the whole Sternum. 



The anterior division, or the body of the posterior sternal bone [_pr(B-mesosternum\ is a mode- 

 rately thick shield-like plate, which is slightly concave on its upper aspect, or the surface turned 

 towards the abdominal cavity, and slightly convex on its lower surface. Sometimes, but exceedingly 

 rarely, as in Phrynosoma Harlanii, it is flat on both surfaces. It usually resembles a lozenge in form, 

 with one angle directed forward and another backward (see Plate IX, st.). It is somewhat longer than 

 broad. The anterior angle is rather rounded, and the posterior so truncated that the bone presents at its 

 extremities a transverse though certainly only a short border. In those Scaly Reptiles, however, in which 

 the body possesses great breadth in proportion to its length, which is very common, the shield-like plate 

 of the Sternum also attains great breadth, especially in its posterior half. The otherwise only short 

 posterior border is then of considerable length, and the whole plate presents a general resemblance to 

 a pentagon with equal sides. A strong approximation to such a form occurs in Agama umbra, Basiliscus 

 mitratus, Stellio vulgaris (see Plate XI, fig. 2), and Grammatophora barbata, yet even in these the 

 posterior border is not so large as the two postero-lateral borders. 



In Phrynosoma Harlanii, in which the length and greatest breadth are equal, and in Moloch 

 horridus, in which the greatest breadth is to the length ns 1 '23 to 1, they have tolerably exactly the 

 form of an equilateral pentagon. Whatever, however, may be their form, their antero-lateral borders are 

 in relation to the hooked collar-bones [the coracoids], and their postero-lateral edges with some of the ribs. 



In the typical Scaly Reptiles the shield-like plate of the posterior sternal bone is usually a little 



