1910.] The Skeleton of Monotremes: Archaic Characters. 149 



(15) In the pro-placentals the parasitic adaptations become more 

 largely intra-uterine as the egg-laying period is delayed. The nutritive 

 contents of the yolk sack are reduced, and the vascular uterine walls finally 

 come in contact with both allantois and yolk sack (e/. Manis). Nutritive 

 material in the maternal blood passes at first by osmosis to the embryo. 

 In the Marsupials it is the yolk sack placenta, in the Placentals, the allantoic 

 placenta which finally predominates. 



(17) Pari passu with the prolongation of intra-uterine development the 

 Placentals give up the imperfect marsupium, vestiges of which seem to be 

 represented in the protractor muscle of the jjrepuce in Carnivora and Artio- 

 dactyla (Weber, 1904, p. 35). 



IV. Reptilian, Archaic and Aberrant Characters in the Skeleton 



of inionotremes, tending to separate them widely from the 



Higher ^Mammals and collectively indicating that they are 



AN EXTREMELY EaRLY OfFSHOOT OF THE MaMMALIAN StOCK. 



Skull. ^ — A. Top and side views. — Chondrocranium unusually massive 

 and well developed (Parker, 1885-86, p. 80). Contrast the fenestrated 

 chondrocranium of most Placentals. 



Frontals facing forward and upward, rather small between orbits (e. g., 

 like Tritylodon). Brain confined to the occipital and parietal segments. 

 (Primitive mammalian character). 



Separate postfrontals. Of these structures Van Bemmelen (1901.2, p. 

 794 says: "... .ein Postfrontale ist zwar nach meinem Befunden vorhanden, 

 aber nicht in dem character eines Deckknochens, sondern als selbstandiger 

 Knochenkern in vorderen Theil der sogenannten Parietalplatte des Prim- 

 ordialcranium." Hence homology with reptilian postfrontal is doubted.^ 

 At any rate unique in mammals. "Prefrontal" in Oniithorhynchus (de- 

 scribed by Seeley) due to overlap of nasals by f^'ontals. Interpreted as 

 orbital portion of frontal by van Bemmelen. 



Broad median parietal homologous (?) with the interparietal of Mar- 

 supials and Placentals. 



Broad squamosal, leaving open canal posteriorly, analogous to post- 

 temporal arcade in Sphenodon and possibly homologous with a similarly 

 placed canal in Anomodonts. Unique in mammals. 



No separate lachrymal (van Bemmelen). (Aberrant character.) 



Jugal absent in Echidna, represented by a vestigial postorbital apophysis 



1 C/. van Bemmelen 1901.2, Taf. xxx-xxxii; Lubosch, 1906, Taf. 26, and text fig. 1, 2. 

 ^Homologized by Luboscli (1906, text flg. 1, .5) with the orbitosphenoid {Ornith.) or 

 orbitosphenoid + ijostfrontal {EcMd.) 



