MONOTREMATA. 



373 



petrous, but separated from the latter, at its 

 lower half, by a fissure analogous to but wider 

 than the canal which traverses the squamous 

 union of the same bones in the Echidna. The 

 base of the squamous plate is contracted but 

 thickened to form the origin of the zygomatic 

 process: it sends inwards a thin plate, concave 

 externally, which forms the glenoid cavity for 

 the lower jaw, and is applied against the basal 

 part of the petrous bone. The glenoid plate 

 is equal in size to the squamous process. They 

 meet at a right angle, and, from the tubercle 

 developed from their union, external to the 

 glenoid cavity, the zygomatic process is con- 

 tinued forwards to join that which is sent off 

 by the superior maxillary bone. I could not 

 find any distinct malar bone in the young 

 Ornithorhynchus. The same arguments against 

 considering the squamous bone to be the malar 

 apply to this Monotreme as have been used in 

 reference to the Echidna. 



The parietal bone (fig. 172, rf) forms the 

 chief part of the upper region of the skull; it 

 is longer in proportion to its breadth than in 

 the Echidna. Both the lambdoidal and co- 

 ronal sutures are squamous, and the parietal 

 overlaps the frontal bones. There is no trace 

 of sagittal suture ; the bony falx is developed 

 from the line which that suture would occupy. 

 The lateral connexions of the parietal differ 

 from those in the Echidna by its union with the 

 squamous portion of the temporal bone, as 

 above-mentioned . 



The frontal bones (/(,/<) are relatively smaller 



than even in the Echidna: they were divided 



by a suture in the specimen described. The 



form of their exposed surface is shown in 



fig. 172, h, h. 



The side of the skull anterior to the petrous 

 bone is formed by the great ala of the sphenoid 

 (fig. 172, i), which is joined by a well-defined 

 linear harmonia to the parietal bone. 



The Ornithorhynchus differs from the Echidna 

 in the large vacuities in the floor of the skull 

 behind and in front of the tympanic cavity, the 

 one representing the combined jugular and 

 condyloid foramen, the other the oval foramen, 

 between which the body of the sphenoid also 

 presents two membranous spaces. The skull 

 differs also in the larger size of the foramen 

 rotundum, in the stronger zygomata, the more 

 complete orbit, and the singular modification 

 of the bones supporting the beak. The cranial 

 cavity is proportionally smaller and shallower 

 in the Ornithorhynchus. The sutures of the 

 cranial bones are much sooner obliterated, and 

 the whole upper and lateral parietes then con- 

 sist of a thin continuous dense plate of bone 

 without diploe. 



The resemblance which the Ornithorhynchus 

 offers in this respect to the class of Birds is 

 noticed by Meckel. 



The oblique canal, (fig. 173, c, r,) which 

 traverses the squamous suture between the 

 petrous and squamous portions of the temporal 

 in the Echidna, is so much shorter and wider 

 in the Ornithorhynchus that it appears to de- 

 tach from the side of the cranium a distinct 



superior column or root (fig. 173, c, a) to the 

 posterior commencement of the zygomatic arch. 

 An analogous canal runs between the tym- 

 panic and mastoid bones in the skull of the 

 Crocodile, and is dilated to great width in the 

 Lizards ; but the presence of a distinct tym- 

 panic bone in the usual position in the Orni- 

 thorhynchus nullifies the supposition that the 

 upper root of the zygoma can be the analogue 

 of the os quadratum in the Ovipara. 



The articular surface for the lower jaw 

 (Jig.l73,c,b) is much more distinctly developed 

 than in the Echidna; it occupies the base of 

 the zygoma, is extended and concave trans- 

 versely, narrower and slightly convex from 

 before backwards : it is not defended by any 

 posterior process. The zygoma is complete, 

 and consists of a nearly vertical straight plate 

 of bone, expanded at its anterior or maxillary 

 extremity, where it sends upwards an angular 

 process bounding the orbit posteriorly, and 

 bends downwards to support the broad alve- 

 olus of the horny molar tooth (Jig. 173, A,g, h). 

 The orbits are small and directed obliquely 

 upwards and outwards. The skull is more 

 contracted between them than in the Echidna, 

 but anterior to them it begins to expand, be- 

 comes flattened horizontally, then bifurcates, 

 and the two depressed branches, after slightly 

 diverging, terminate each by an inwardly 

 inflected process, the extremities of which 

 are half an inch apart. The space inter- 

 cepted by the facial fork is the external bony 

 nostril (Jig. 172, p), which is thus left incom- 

 plete anteriorly. The forms and pioportions 

 in which the bones of the face enter into the 

 formation of the external nasal aperture are 

 illustrated in the figure of the cranium of 

 the Ornithorhynchus given by Pander and 

 D'Alton,* and I have verified the accuracy 

 with which the sutures are there delineated. 

 Cuvierf supposed the facial forks to be formed 

 by the intermaxillary bones, and describes a 

 small bone in the middle of their interspace 

 (fig. 172, p) suspended in the cartilage of the 

 upper mandible, and with an emargination on 

 each side of its inferior plate, which he conjec- 

 tured might represent the nasal bones and the 

 palatine part of the intermaxillary bones. The 

 true nasal bones (fig.\7'2, n, n) are, however, 

 as shown by Pander and D'Alton, analogous in 

 situation to and more largely developed than 

 those of the Echidna. They commence each by 

 an angular process, which overlaps the frontal, 

 and extends into the inter-orbital space. They 

 are continued forward of equal breadth, and 

 have their anterior extremity obliquely trun- 

 cated arid terminated in a fine point, which 

 extends to the middle of the inner side of the 

 facial fork. The nasal bones thus form the pos- 

 terior half of the boundary of the bony nostril. 

 The superior maxillary bone (fig. 172, in, in), 

 after sending oft' a process (c), which curves, 

 over the ant-orbital foramen, extends forwards 



* Skelete der zahnloscn Thiere, 1825, (tab. ii. 

 fig. a.) 



t Li^ons d'Anat. Comp. 1837, ii. p. 455. 



