206 SHOULDER-GIRDLE AND BREAST-BONE. 



and behind; in front the prge-sternum is narrowed and square between the clavicles in T. peba; 

 in E. villosus it is but little produced, and is emarginate at this part. It dilates greatly between 

 the first sternal ribs (s. r.), which are extremely broad, and articulate with the rnanubrium by an 

 oblique, oblong synovial cavity. Behind the first ribs the manubrium gently lessens to two-fifths 

 of its width to again articulate with the first meso-sternal ; there are four of these (figs. 1 and 12, 

 m. st.), which gradually lessen towards the xiphoid cartilage. This latter part (x.) is very elegant 

 in T. peba, being very much like what is seen in certain Anoura ; it is like a cheese-knife, but 

 the " blade " has notched retral " ears." The sternal segments are nearly flat at the top, and con- 

 tract rapidly, being sub-carinate below (fig. 14, E. villosus, three diameters ; second meso-sternal 

 from behind, and section of third sternal ribs) ; at this stage the joint-cavities are continuous. 

 Of the two embryos, that of E. villosus was ripest (both nearly ripe) but in it the first pleuro- 

 steon only, had appeared (fig. 12) : in the second joint the vascular puncta were seen. In T. peba 

 there were three pleurostea ; a section through the Sternum shows (fig. 6, plo., five diameters) 

 that the osseous growth is internal : there is a nail-shaped "metostcon" in the xiphoid (x.). In 

 T. peba there is a commenced transverse cleft between the vertebral and sternal ribs (fig. 1, 

 v. r., s. r.) ; this has not yet appeared in E. villosus ; and in that species the posterior sternal 

 ribs are of great breadth, as in the Echidna. In a three-quarters-grown T. peba (fig. 7, one and 

 a half diameters) the front or coracoid part of the prse-sternum is lessened, and is only partly 

 ossified by the first pleurosteon, which also leaves its costal and posterior regions untouched : in 

 these, however, epiphyses appear ; one for each costal facet ; one behind for the first meso-sternal . 

 and one on each side for the semi-facets for the second sternal ribs. When the meso-sternals 

 are viewed endwise (fig. 8, T. peba, three-quarters adult ; first meso-sternal, from behind, two dia- 

 meters) it is then apparent that there is an additional pleurosteon in each of the five cartilaginous 

 facets, at each end of the segment : for the sternal ribs get a double head ; each head articulates 

 with a semi-facet of two sternal pieces ; and every such cartilaginous face has an epiphysis within 

 it. The main facet, that for the corresponding part of another sternal segment, is sub-triangular, 

 and this has the largest epiphysis within it. Altogether, each sternal segment has in it eleven 

 osseous centres ! The articulation of these quasi- vertebra3 is " opisthoccelian ;" the last meso- 

 sternal is sometimes bi-convex, however (see fig. 15, m. st., Prionodos ffiffas, two-thirds natural 

 size), and the xiphoid may be altogether wider than the meso-sternals (fig. 15, x.) : the outspread 

 end of the xiphoid remains soft. In this same kind (Prionodos) the manubrium (fig. 16, ideal 

 section, two-thirds natural size) is strongly keeled ; and, as the bony matter of the great " pleur- 

 osteon " is slow to reach the bottom of the keel, this part is finished by a second endosteal 

 patch, which has the most claim of any Mammalian sternal bone I have seen to be considered 

 the counterpart of the ornithic keel-bone ("lophosteon"). 



It is possible that a small endosteal sternal rib may appear in the first costal arch in E. vil- 

 losus (see fig. 12, s. r. 1) ; at any rate, the first vertebral rib (v. r. 1) reaches the Sternum below. 

 In the two-thirds adult T. peba, such an osseous centre appears (fig. 3, s. r. 1); in the same 

 specimen the "costa intermedia" is well shown (fig. 4, i. r., sixth right rib, nat. size); indeed, 

 there are three centres at this part, above the synovial joint (sy.) : the same rib in a very old indi- 

 vidual (fig. 5) has lost all traces of this beautiful outcropping of a Reptilian character. A rib from 

 the middle of the chest in Xenurus imicinctus (fig. 9, nat. size) shows no trace of this segment ; this 

 character, however, is variable in the same individual, as may be seen in rather old specimens of 

 Dasypus sexcinctus (Euphractus setosus), for in the last fully developed rib but two (fig. 10, v. r., 



