191U.] The Skeleton of Moiioiremefi: Archaic Characters. 151 



X, XI) (cf. van Beramelen, pi. xxx). In Echidna these nerves pass out 

 through an obUque fissure above the mastoid process. 



In both genera the optic foramen is iniited with the foramen lacerum 

 anterius (sphenorbitale) as in JNIarsupials. The foramen rotundum (V,) 

 is distinet in OrnifhorJii/ncJiu.s l)ut united with the S{)henorbital fissure in 

 Echidna. 



As stated below (p. 427) in OrnitJiorlnjiichns there is only a single olfac- 

 torij foramen instead of a cribriform plate (unique; aberrant) while in 

 Echidna the plate is present and is horizontal. (Aberrant ?.) 



There is no osseous arch above tJie external auditory meatus (i. e., no 

 post-tympanic process of the scjuamosal. (x\berrant.) In Ornithorhynchiis 

 the small romid tympanic opening, at the bottom of which lies the fenestra 

 vestibuli sen ovalis, is antero-internal to the prominent mastoid process of 

 the periotic. In Echidna the tympanic fossa (the roof of the tympanic cavity) 

 is a large triangular depression bounded posteriorly by the prominent trans- 

 verse mastoid process and antero-internally by the flattened pterygoid. 



The tympanic opening is approximately horizontal. Weber (1904, p. 

 320) suggests that this may not be interpreted as a primitive character with- 

 out some inicertainty, on account of the degenerate nature of the lower jaw 

 in both genera ; but the horizontality of the tympanic ring in many embryo 

 mammals (cf. Parker, 1886) and in certain adult Marsupials and Insectivores, 

 together with the fact that the tympanic cavity arises below and internal to 

 the tympanic ring, establishes the presumption that the horizontal position 

 is a primitive one. 



Auditory hones. — Tympanic ring more or less semicircular, not expanded 

 (primitive); in Echidna touching the temporal process of the pterygoid. 

 The latter character, if the tympanic represented the transformed quadrate 

 (Gadow, see p. 128 above) would no doubt be regarded as primitive, but it 

 seems more likely to be the result of the extreme backward prolongation of 

 the palate, with the consecjuent posterolateral displacement of the pterygoids 

 {cf., van Bemmelen's Taf. xxxi). Tympanic ring connected with very large 

 anterior or Folian process of the malleus (Primitive, cf. the figures of Mar- 

 supials and embryonic Edentates and Insectivores given by Doran, 1879, 

 and Parker, 1886). Ossicula auditus relatively large (primitive); malleus 

 firmly connected with incus (aberrant) ; stapes columelliform, imperforate 

 (primitive). Cochlea, very primitive, not spirally wound but turning only 

 through 180° (Denker, 1901). 



For further notes on the Monotreme skull see pp. 155, 156. 



