374 



MONOTREMATA. 



in a pointed form along the outer side of the 

 facial fork as far as the nasal does on the inner 

 side, and an angular fissure is intercepted 

 between the anterior extremities of these bones 

 into which the pointed posterior part of the inter- 

 maxillary bone (jig. 172, 0,0) is inserted. The 

 anterior half of the facial fork with its inflected 

 end is wholly formed by the intermaxillary 

 bones, which thus bound the anterior half of 

 the wide external nasal aperture. The small 

 detached intermediate bone (Jig. 173, A, 6) may 

 be regarded as a separate centre of ossification 

 of the palatine process of the intermaxillaries, 

 and of the middle stem which divides the ante- 

 rior nostrils in birds and lizards. 



The vomer forms a bony septum, dividing 

 the whole extent of the nasal canal from the 

 spine of the sphenoid forwards. 



There is a small lachrymal foramen (fig. 

 169, I) at the anterior and inner part of the 

 orbit in both the genera of Monotremes; a 

 little lower down is the commencement of the 

 ant-orbital canal. This canal branches in the 

 Echidna, and terminates on the outer side of 

 the maxillary bone by a succession of small 

 foramina; but in the Ornithorhynchus, where 

 it transmits a much larger sensitive nerve, it 

 divides into three canals, of which one emerges 

 beneath the uncinated process of the maxillary 

 above mentioned ; a second descends and opens 

 upon the palate ; and the third passes forwards 

 into the substance of the facial fork, and termi- 

 nates by a large foramen at the outside of the 

 intermaxillary bone. 



On the exterior of the cranium the ridges 

 indicating the extent of the temporal muscles 

 are clearly developed in the Ornithorhynchus, 

 and correspond with the stronger zygomata and 

 the more complete apparatus for mastication 

 in this Monotreme. Four linear impressions 

 upon the upper surface of the skull diverge 

 from the middle of the lambdoidal ridge, and 

 terminate at the temporal ridges. The occipital 

 foramen (fg. 173, c) has a vertical plane, as in 

 the Echidna, and has a similar rounded notch 

 at its upper part. 



The interior of the skull offers many unusual 

 modifications. The sella turcica is elongated 

 and narrow in both Monotremes ; it is bounded 

 by two very distinct lateral walls in the Echidna. 

 The posterior clinoid processes are chiefly 

 remarkable for their height in the Ornitho- 

 rhynchus. The semicircular canals stand out 

 in high relief in this species, as in Birds. In 

 the Echidna the ethmoid encroaches upon the 

 anterior part of the cranial cavity in the form 

 of a large convex protuberance made by the 

 posterior wall of the olfactory cavity, and a 

 very extensive cribriform plate is developed. 

 In the Ornithorhynchus the olfactory tract is 

 comparatively small, in the form of a depres- 

 sion, and the nerve escapes by a single foramen 

 at the anterior part of the ethmoidal plate. 

 This is likewise an interesting mark of affinity 

 to the bird and reptile ; but the most remark- 

 able and characteristic feature in the interior 

 of the skull of the Ornithorhynchus is the bony 

 falx (fig. 173, B). This is not present in the 



Echidna. The tentorium is membranous in 

 both Monotremes. 



The lower jaw consists in the Echidna of 

 two long and slender styliform rami without a 

 symphysial joint, but loosely connected together 

 at their anterior extremities. An angular pro- 

 cess divides the horizontal from the ascending 

 ramus, which rises at an open angle and ter- 

 minates in a small oblong convex condyle. A 

 short obtuse coronoid process extends from the 

 upper part of the horizontal ramus as far in 

 advance of the angle as the condyle is behind 

 it. The rest of the ramus is rounded like a rib, 

 and diminishes to the anterior extremity. The 

 dental canal commences below the coronoid 

 process and divides in its progress, one branch 

 terminating near the middle of the smooth 

 alveolar border, the other close to the end of the 

 ramus. In no Mammiferous animal does the 

 lower jaw bear so small a proportion to the skull 

 or to the rest of the skeleton as in the Echidna. 



In the Ornithorhynchus the lower jaw is 

 much more developed (jig. 173, E). Each 

 ramus commences posteriorly by a large sub- 

 hemispherical condyle, the convexity of which, 

 so characteristic of the Mammalian class, is 

 strongly marked. The ascending ramus is 

 nearly horizontal, flattened below, and con- 

 tinued upwards in the form of a low vertical 

 compressed plate, on each side of which there 

 is a deep fossa. The ascending is continued 

 by a gentle curve into the horizontal ramus, 

 and the angle of the jaw is very feebly indicated. 

 The horizontal ramus suddenly expands and 

 sends off above in the same transverse line 

 two short obtuse processes, both of which 

 might be termed ' coronoid ;' this structure is 

 peculiar to the Ornithorhynchus. The inner- 

 most process (c), although the largest, is the 

 superadded structure, as it affords insertion to 

 the internal pterygoid. About two lines anterior 

 to these processes the upper border of the 

 horizontal ramus expands to form the shallow 

 oblong alveolus (e) for the horny grinder. Its 

 floor is perforated by several large foramina. 

 The dental canal divides ; one branch opens by 

 a wide elliptical foramen on the outside of the 

 ramus immediately anterior to the alveolus, 

 the other terminates at the lower part of the 

 end of the ramus. The rami of the jaw con- 

 verge and are united at a symphysis of more 

 than half an inch in length; there they become 

 expanded and flattened, then again disunite, 

 and are continued forwards as two spatulate 

 processes (b), which diverge from each other to 

 their broad rounded terminations, and are 

 situated just behind the inflected extremities of 

 the similarly separated inter-maxillaries (Jig. 

 173, A, c, r). On the outer sides of the upper 

 surface of the broad symphysis are the long 

 and narrow sockets of the two anterior trenchant 

 horny teeth. The Monotremes differ from the 

 Marsupials in the absence of the inflected pro- 

 cess developed from the angle of the lower jaw. 



Of the vertebral column. Both Monotremes 

 have twenty-six true vertebrae, of which the 

 seven first are cervical. In the Echidna sixteen, 

 and in the Ornithorhynchus seventeen, of the 



