150 Bulletin American Museum of Natural History. [Vol. XXVII, 



of the zygomatic process of the maxillary in OrnitliorJiynchus. (y-M\ 

 Bemmelen, /. c, Taf. xxx.) (Aberrant.) 



Zygomatic process of squamosal extending back of glenoid (Primitive). 

 Glenoid fossa in Ornitliorhynchus external to auditory region (r/. Cynodon- 

 tia.) 



Septo-inaxiUarij separate in embryo (Gaupp, 1905). A reptilian char- 

 acter retained also in Dasypus (Broom, 190G. 1, p. 370.) 



Very large mastoid portion of periotic, analogous in position to opisthotic 

 and epiotic of reptiles, extending dorsally as a broad plate postero-lateral to 

 the parietal and in Ornitliorhynchus extending beneath glenoid fossa of 

 squamosal. (Primitive.) The periotic complex consists of three parts: 



(1) mastoid, exposed on postero-lateral part of brain case external to parietal; 



(2) mastoid process, seen in palatal view posterior to the tympanic fossa; 



(3) petrosal, lodging the semicircular canals, etc. 



B. Palatal view. — The dumb-bell bone or prevomer of OrnitliorJiynchus 

 is equivalent to the palatine process of the premaxilla in other mammals 

 (Broom, 1903.1), to the anterior paired vomer of foetal Insectivores, etc. 

 .(W. K. Parker, 1886) and to the vomer of Lacertilia and Ophidia (van 

 Bemmelen, 1901, p. 753). (Primitive.) 



Posterior naves in both genera extended backward further than in any 

 other mammal except Myrmecophaeja. (Partly secondary (?).) 



Pterygoids. According to Gaupp (1905) the flattened pterygoids of 

 Monotremes are homologous with the elements of the same name in Saurop- 

 sida, but not with the so called pterygoids of other mammals which he calls 

 the "parabasale" and homologizes with the lateral wings of the parasphenoid 

 (presphenoid rostrum) of reptiles. A comparison of the pterygoids of foetal 

 Monotremes (van Bemmelen, 1901, i)l. xxxi, xxxii) and of foetal Edentates 

 and Insectivores (Parker, 1886) fails however to convince the writer that the 

 pterygoids of Monotremes have any different homology and derivation from 

 those of Placentals (c/. also Cynognathus p. 120). 



The palatine and pterygoid enter into the floor of the brain-case (Aber- 

 rant) . 



Occipital condyles with a median basiocci})ital portion {i. e., tripartite, 

 cf. Cynodontia). Possibly a fossorial adaptation. 



Carotid foramen. — The entocarotid {cf. van Bemmelen, 1900.2, plate; 

 1901.2, pi. xxx, xxxi) pierces the floor of the cranium at the posterior end of 

 the basisphenoid, an arrangement which approximates more closely to the 

 Marsupial than to the Placental condition. 



Occipital fenestra. In Ornithorhynchus in front of the occipital condyle 

 on each side is a large fenestra which corresponds with the combined foramen 

 condylare (nerve XII) and foramen jugulare (for. lac. post., nerves, IX, 



