194 SHOULDER-GIRDLE AND BREAST-BONE. 



distinct crest on the post-scapula (sc. c.) ; the notch between the meso- and the post-scapula is 

 sharp and deep, and the thick base of the scapula is seen to form about one fourth of the glenoid 

 cavity (gl.). This cavity is much larger than in Ornithorhynchus ; the part of the coracoid (cr.) 

 forming this is higher and thicker; and the rest of the bone is much more solid and tuberous : in 

 its clubbed posterior angle there is an endosteal " post-epicoracoid " (p. e. cr.) . The epicoracoid 

 proper (e. cr.) is less than in the other type, and the right overlaps the left : all the bones forming 

 the Shoulder-girdle moiety are seen to be mere osseous regions of one common cartilaginous 

 mass. The clavicles (fig. 11, cl.), and the inter-clavicles (i. cl.), are seen to be turned upwards to 

 a greater degree than in the Platypus, and the pointed lower ends of the former lie further 

 backwards on the median splint ; this keystone piece has its rami larger and its body smaller 

 than in the other kind. The sternal piece found in the interior half of the manubrium (" prae- 

 sternum ") is, in this case, an ectosteal patch, and such an investment is also seen on the first 

 (s. r. 1), as well as in the other sternal ribs. Fig. 10 (which is from a very young individual, 

 and magnified one and a half diameter) shows all the ectosteal patches, save the pro-osteon ; 

 this is seen in the adult (figs. 11 13, pr. o.). The lunate ectosteal "pleurostea" are very 

 symmetrical in the young (fig. 10, pi. o.) ; but in the large specimens they become very 

 irregular (figs. 12, 13, pi. o.). Pig. 14 shows the synovial facet of the front of the third 

 meso-sternal piece in an individual one-third adult, and also the synovial cavities between it and 

 the sternal ribs. In fig. 10 is seen the thin laminar character of the clavicles (cl.), even at their 

 upper ends ; this shows that they are simple as in the Reptiles. In the development of an 

 endosteal " costa intermedia," in the absence of a free xiphisternum, and in the huge size of the 

 hinder, free sternal ribs, the Echidna agrees very closely with the Ornitkorhynchus. I may 

 remark, in passing, that developmentally there is no absolute necessity for the existence of such 

 a Class as that formed by the Birds, for the Monotremes form a very easy stepping-stone from the 

 Reptiles to the Mammals, which are, indeed, the true culminating forms that arise, as it were, 

 from the scaly quadrupeds. As for the Birds, I shall be able to show, in other papers, that 

 in the most important morphological regions, namely, the skull and the face, they repeat with 

 minute accuracy many of the characters of the Osseous Fishes. 



