370 



MONOTIIEMATA. 



by a squamous modification of the coronal 

 suture. The part described in this Article 

 as a lamelliform portion of the petrous bone, 

 (jig. 169, e,) which extends upon the lateral 

 and part of the posterior region of the skull, is 

 regarded by the Editors* of the Lecons d'Ana- 

 tomie Comparee, Ed. 1837, as the squamous 

 portion of the temporal ; and the flat oblong 

 bone, (Jig. 169, B,) which forms part of the la- 

 teral wall of the cramal cavity and the posterior 

 half of the zygomatic arch, and which supports 

 the articular surface for the lower jaw, is thought 

 to be the malar bone. But when we consider 

 the low development or total disappearance of 

 the malar bone in the skull of the Insectivora 

 generally, as in Echinops and Centetes among 

 the Fera, and as in the edentate Munis and 

 Myrinecoph(iga> it is unlikely that the malar 

 bone should attain so superior a size and fulfil 

 such important functions in the Monotrema- 

 tous Edentata, in which its condition, according 

 to the above views of the editors of the Lecons 

 d'Anat. Comparee, would be unique in the 

 rnammiferous class. It appears to me to be 

 more reasonable to regard the malar bone as 

 either altogether absent in the Echidna, as it is 

 in the Manis, and the zygomatic arch as being 

 completed in the Echidna by a greater exten- 

 sion of the zygomatic processes of the tem- 

 poral and superior maxillary bones; or else to 

 suppose that these are actually united, at an 

 earlier period, by a separate intervening jugal 

 style, which, however, I have not been more 

 successful in finding than the Continuators of 

 Cuvier. With respect to the great develop- 

 ment which the petrous bone, according to my 

 view, must present in the Echidna, it may be 

 observed that this bone forms part of the occi- 

 pital region of the skull in most Marsupials, 

 and also contributes as large a proportion to 

 the lateral parietes of the skull in certain Ro- 

 dents, as the Helamys, as it is here described 

 to do in the Echidna. 



The side of the cranium is principally formed 

 by the largely developed petrous bone (Jig. 170, 

 e) and the great ala of the sphenoid, which meet 

 and are joined in the interspace separating the 

 parietal from the sqnarno-temporal bone, by a 

 nearly vertical harmonia half an inch long ; 

 this harmonia is crossed at nearly right angles 

 by a deep groove, which in some parts sinks 

 through the wall of the cranium and exposes 

 its cavity. The groove or canal first runs be- 

 tween the squamous and petrous elements of 

 the temporal bone, and is a conspicuous fea- 

 ture in the skull of the Ornithorhynchus. 



The lower part of the side of the skull of the 

 Echidna is closed by the squumous element of 

 the temporal, which overlaps a large portion 

 of the petrous bone, and by a small portion of 

 the sphenoid : it is represented detached from 

 the skull at fig. 169, B. The lower boundary 

 of the squamo-temporal forms a straight line, 

 from which the glenoid surface for the lower 

 jaw (f) is extended inwards at a right angle, 

 upon the base of the skull; the anterior part 



The very able anatomists, MM. Laurillard and 

 '- 



is continued forwards, protecting the temporal 

 fossa by a thin vertical plate of bone, and then 

 diminishes to a slender, straight, styliform, 

 zygomatic process which rests obliquely on a 

 corresponding process of the superior maxil- 

 lary bone. 



The tympanic cavity is shallow, and exca- 

 vated in the basal part of the petrous bone, 

 where it is widely open in the macerated skull : 

 it is figured closed by the tympanic bone and 

 membrane at g,fig- 170, and exposed by their 

 removal at e" Jig. 170. The plane of the mem- 

 brana tympani is horizontal, and its exter- 

 nal surface looks nearly downwards. Three- 

 fourths of its circumference are implanted in 

 the groove of the very slender incomplete hoop 

 formed by the detached tympanic bone, which 

 is figured with the anchylosed malleus at c,Jig. 

 169. The petrous bone is continued from the 

 tympanic fossa forwards and inwards, in the 

 form of a broad and nearly horizontal process, 

 (Jig. 170, e',) to the pterygoid plate of the sphe- 

 noid, (i, i',) which is also horizontal. The pe- 

 trous and pterygoid plates are joined by an 

 oblique harmonia, and both contribute to extend 

 the bony palate backwards. The palatal pro- 

 cess of the petrous bone is abruptly terminated 

 behind by the Eustachian groove (Jig. 170*). 



The frontal bone (Jig. 169, //) in the cranium 

 here described was divided by a median frontal 

 suture, toothless like the rest ; the angle be- 

 tween the superior and the orbital plates is 

 rounded off; the orbital plate joins the great 

 ala of the sphenoid by a deeply sinuous suture. 

 The anterior part of the frontal is largely over- 

 lapped by the bases of the nasal bones, which 

 encroach upon the interorbital space. 



The nasal bones (Jig. 169, n) receive the upper 

 edge of the superior maxillary bone into a groove 

 at their outer margin, and articulate anteriorly 

 with the intermaxilluries (Jig- 169,o); but these 

 meet above the nasal canal in front of the nasal 

 bones for an extent of about three lines, and 

 thus exclusively form the boundary of the single, 

 oval, and terminal external nostril. The lower 

 or palatal process of the intermaxillary extends 

 backwards in the form of a long and slender 

 pointed process which is wedged into a fissure 

 of the superior maxillary bone. 



The anterior palatal or incisive foramen is a 

 single large elongated fissure extending from 

 the narrow anterior symphysis of the inter- 

 maxillaries backwards, for some distance, be- 

 tween the palatal processes of the maxillaries. 

 At the back part of the bony palate a narrow 

 fissure extends forwards between the pterygoid 

 bones, and the intermediate exient of the 

 bony palate is entire, or presents only a few 

 minute perforations. The palatal bones, if 

 originally distinct, soon become confluent with 

 the maxillaries. There was no separate osseous 

 style representing the malar bone between the 

 zygomatic processes of the maxillary and tem- 

 poral bones in the skull here described.* 



The zygomatic process of the superior max- 



* Cuvier says, " Entre ces deux apophyses est 

 un tres-petit filet qui represent le jugal." Cuvier, 

 Ossem. Foss. 4to., vol. v.,p. 145. 



