AND IRREVERSIBLE EVOLUTION. 1047 



In spite of many mammalian characters, such as the structure 

 of the teeth, the articulation of the lower jaw, and the shape of 

 the bi-ain, not the Cynognathiclie but the Bauriamorpha must be 

 considered as the ancestors of the Mammalia, for the ribs of the 

 former show a non-mammalian trend of evolution.' Curiously 

 enough the Bauriamorpha have no postorbital bar. Thur-s the 

 disappearance of this part in the Bauriamorpha and its reappear- 

 ance in the higher mammals again points towards a reversal. 

 Wortmann's discoveries of a separate postfrontal and even of a 

 postorbital bar in some Insectivora (19) show tliat tliis part of 

 the mammals is not analogous but homologous with the same 

 pai't in reptiles. 



This change seems again to bo nothing else than the reten- 

 tion of an embryonic character in the adult, for frequently 

 in embryos of animals characters appear that aie later reduced. 

 Good examples are afforded by the temporary development of a 

 third cervical rib in the Lacertilia (8) and by the development 

 of a fourth and fifth digit in embryos of birds (13). 



A process similar to that which accounts for the development 

 of the postorbital bar in higher mammals is evidently also 

 changing the development of the claws in Ojnsthocomns, for 

 this bird is evidently forgetting how to Hy and learning how to 

 climb (10). 



For the history of the development of the postorbital bar in 

 llcj)tilos and Mammals the following diagram can be drawn : — 

 I'oslorbUal bar comjilete. I'oslorhilal bar inoomjiletc. 



Piiinitivo Tlieriodoiits - - >^Bnuri:iinov)ilia (ov similiir 



Thcriodoiits). 



' .. .^ i 



Specialised Mammalia-< Primitive ]\Iamnialia. 



§ (3) The develojjment of the ventral elements of the pelvis. 

 As is well known, in primitive Stegocephalians, for example 

 the Branchiosauridse, the ventral elements of the pelvis consist 

 of four, or sometimes even only of two, small disk-shaped centres 

 of ossification that were evidentlj' embedded in a large plate of 

 cartilage. Much the same type of pelvis is found in the recent 

 TJrodeles, In the more specialised Stegocephalians {Eryo2>s, 

 Oacojjs) the two ventral elements form a continuous mass of 

 plate-like bone with a small foramen perforating each pubis. It 

 is evident that this type of pelvis originated in the complete 

 ossification of the whole cartilage of the more primitive forms. 

 This solid type occurs also in the Cot3dosauiians {Seymouria, 

 Diadectes, Labidosaurus (text-fig. 8 (1)), Pweiasnurus) ] in the 

 most agile Cotylosaurians (Procolophoa), however, and in the 

 Pelycosaurians (Ophiacodon, Varanoscmrus) a central perforation 

 and separation of the pubis and ischium appear. From this 

 latter t3'pe were evolved the pelves of the higher reptiles, that 

 «how either one groat perforation in the centre and two small 

 foramina obturatoria passing through the pubes, or one large 



68* 



