MONOTREMATA. 



371 



illary bone (Jig. 169, m) extends backwards as 

 far as tlie posterior boundary of the zygomatic 

 or temporal fossa; the palatal process extends 

 along the floor of the orbit in a similar form 

 and to nearly the same extent. The orbit is 

 marked off from the temporal fossa by merely 

 a slight ridge extending down and across the 

 suture joining the frontal and sphenoid bones. 



The skull of the Echidna differs from that of 

 the edentulous Manis and Myrmecophaga in 

 the completion of the zygomatic arches, in the 

 unclosed state of the tympanic cavity, in the 

 large size of the foramen incisivum, and the 

 surrounding of the external nostrils by the 

 intermaxillary bones alone : it differs also in 

 the smaller relative distance between the poste- 

 rior palatal fissure and the superior maxillary 

 bones, and in the apparent absence of the pala- 

 tine bones, the presence and interposition of 

 which between the pterygoid and maxillary 

 palatal plates elongates the palate in the pla- 

 cental Anteaters at the part where it is rela- 

 tively shorter in the Echidna. In the modi- 

 fication of the pterygoid plates of the sphenoid 

 to complete the posterior nasal canal, the 

 Echidna manifests an interesting resemblance 

 with the great Anteater ; but it differs from 

 this, as from every other mammiferous species, 

 in the palatal plates contributed by the petrous 

 bones to the broad posterior part of the roof of 

 the mouth which supports the horny palatal 

 teeth. Cuvier describes the posterior palatal 

 fissure as extending between the palatine bones, 

 and therefore regards the plates, which are here 

 affirmed to be developed from the petrous bone, 

 as being the pterygoid processes of the sphenoid ; 

 and, according to this view, he truly observes 

 that their horizontal position is very remark- 

 able;* but he might have added, that their 

 share in the formation of the tympanic cavity 

 was not less so. The same determination of 

 the bones composing the posterior part of the 

 osseous palate of the Echidna is repeated in 

 the Lecons d'Anatomie Comparee, 1837, 

 p. 454. If, however, the sphenoid be sepa- 

 rated from the occipital bone, which was easily 

 done in the young skull of the Echidna repre- 

 sented h\/?gs.l69and 170, the horizontal plates, 

 described by Cuvier as pterygoids, are left 

 behind, not as separate bones, but as conti- 

 nuous portions of the petrous elements of the 

 temporal, which form, at the same time, part of 

 the base of the cranial cavity, complete the 

 inner wall of the tympanum, and the anterior 

 part of the Eustachian groove. The palatines 

 of Cuvier are developed from the sides of the 

 basi-sphenoid, and almost immediately bend 

 inwards and meet below the nasal canal, which 

 they thus prolong posteriorly, as in the Myr- 

 mecopfuiga ; and they are separated poste- 

 riorly, also, as in that genus, by an acute fissure, 

 presenting unequivocally the same modifications 



* Cuvier says, " Une echancrure digne et pro- 

 fonde separe les palatins en arriere. Le plan de 

 chacun d'eux est continue en dessous par une apo- 

 physe pterygoide, qui ici, chose bien remarquablc, 

 est horizontale ou a peu pres : elle contribue a 

 former la cavite de la caisse." Ossem. Fossiles, 

 1. c. p, 146. Meckel is silent on this subject. 



which characterize the pterygoids in the plu- 

 cental Anteaters, and in the Crocodile. The 

 suture dividing the pterygoids from the pala- 

 tines iu the Echidna is obliterated, if it ever 

 existed ; or the true palatines may be confluent 

 with the palatine processes of the maxillary 

 bones. 



Some of the Marsupials, as the Wombat, 

 resemble the Echidna in the open state of the 

 tympanic cavity in the dry skull ; but the most 

 essential points of correspondence with the 

 cranial anatomy of the Echidna are found, as 

 might be expected, in the Ornithorhynchus. In 

 this Monotreme the tympanic cavity (fig. 173, 

 A, k) is a simple excavation at the under part of 

 the petrous bone; the periphery of the opening, 

 which looks almost directly downwards, is en- 

 compassed by the tympanic and malleal bones, 

 (Jig. 173, D, o, b,) the outer and anterior part of 

 the circle being formed by the os tympanicum. 

 The tympanic cavity is relatively smaller, but 

 is defended posteriorly by a larger process sent 

 downwards by the petrous bone near its outer 

 side. The petrous bone here forms no palatal 

 process, but the bony roof of the mouth is ter- 

 minated by the pterygoid plates, (Jig. 173, A, 

 i,) which meet below the nasal canal, as in the 

 Echidna, but are not divided by any posterior 

 fissure. In a skull of the Ornithorhynchus, in 

 which the suture dividing the palatine processes 

 of the maxillary bones from the bony palate 

 posterior to them remains, there is no trace of a 

 division between the pterygoid and palatine 

 bones, which contribute to complete the osseous 

 palate (fg. 173, A, e). 



The occipital bone of 

 the young Ornithorhyn- 

 chus corresponds with 

 that of the Echidna in the 

 relative size and position 

 of its four component 

 parts. The ex-occipitals 

 are shown at jig. 172, 

 6, 6, and the supra-occi- 

 pital at c. The petrous 

 element of the temporal 

 (e) likewise sends a thin 

 plate to form the poste- 

 rior part of the side of the 

 cranium, but it does not 

 intervene between the pa- 

 rietal bone and squamous 

 part of the temporal, as 

 in the Echidna. The 

 middle of the upper mar- 

 gin of the cranial plate Skull of young Ornitho- 

 of the petrous bone is rhynehm. (Original.) 

 notched, and a small vacuity here intervenes 

 between the petrous and parietal bones, which is 

 closed by the squamo-temporal (f), the upper 

 margin of which overlaps the descending pos- 

 terior angle of the parietal bone (d). The form 

 of the sqiiamous element of the temporal is 

 very remarkable in the Ornithorhynchus: its 

 ascending cranial or proper squamous portion 

 is a small sub-quadrilateral bony scale, narrower 

 antero-posteriorly, but higher than the corres- 

 ponding part in the Echidna ; articulated by 

 its squamous margin with the parietal and 



2 B 2 



Fig. 172. 



