192 SHOULDER-GIRDLE AND BREAST-BONE. 



CLASS" MAMMALIA." 



Ordo" MONOTREMATA." 



Examples. Ornithorhynchus ; Echidna. 



IN this, the lowest group of the highest Class, we have Reptilian characters in a more 

 remarkable degree than in any existing Bird. These characters, moreover, do not take the 

 observer back merely to the highest types (for instance, the Crocodiles), where there is the 

 most evident adumbration of the Mammal, but to the ordinary Lacertilia, and to the huge 

 extinct Ichthyosaurs. As in the Reptilia and the true Struthionidse, the moieties of the 

 Shoulder-girdle undergo no perfect segmentation, the division of the Shoulder-plate being 

 merely marked out by distinct ectosteal tracts ; the form also of the plate is much more Reptilian 

 than in the Bird. In Ornithorhynchus paradoxus (Plate XVIII, figs. 4 9), the scapula 

 (sc.) is like a pruning hook, being moderately broad and greatly decurved ; its upper or 

 supra-scapular border (s. sc.) is merely slow in its ossification, but there is very little 

 differentiation of it as a region : this agrees with the Bird, and with many of the Mammalia. The 

 posterior margin of the scapula is thickened, and so is the anterior ; this latter is the meso- 

 scapular region, and it sends out a distinct bar below the meso-scapular bar or " acromion ;" the 

 whole of this region is out-turned : there is no prae-scapula. The glenoid cavity (gl.) is shallow, 

 oblique, and Reptilian, and in the old individual the coraco-scapular suture is not seen in its 

 fundus, for the two main bones have totally coalesced, as they do in the Struthious Birds. The 

 coracoid, as a whole, is a broad semilunar blade, with a short, thick shaft (fig. 6, cr., e. cr.), and the 

 segmentation it undergoes is different from what is seen in the Reptilia. The posterior hook is 

 but little developed, and the main shaft bone runs through nearly to the base, behind, but the 

 anterior hook (seen in many Birds at the base of the coracoid) is very largely developed indeed : 

 this part is not feebly ossified by endostosis, as in the Reptiles, but has its own ectosteal centre. 

 A small margin of soft cartilage is left to the main coracoid (cr.), and a larger, thin edge to the 

 epicoracoid (fig. 6, e. cr.) : the left of these overlaps the right. This second, later ectosteal tract 

 continues distinct from the earlier posterior bone (cr.). An elegant oval notch, witn a small 

 opening in front, is seen between the concave lower margin of the meso-scapula and the concave 

 upper margin of the epicoracoid ; this is the " coraco-scapular notch" (c. s. n.) ; it is converted 

 into a sort of " fenestra" by the dermal bones which close the gap. There is no prse-coracoid 

 band of cartilage, but the epicoracoid hook runs far up into that region, besides entrenching upon 

 the meso-coracoid region below, which, like the prae-coracoid, is not here defined. The ectosteal 

 epicoracoid is seen in the Monotremes for the first and the last time, unless it should turn up 

 in some fossil forms ; it is as interesting to the morphologist as the ectosteal supra-scapula 

 of the Anoura. 



But the endoskeletal Shoulder-arch of the Ornithorhynchus is but half complete by itself, 

 for it is strongly undergirt by three " prse-thoracic" dermals, which have cropped up again from 



