C)42 ARISTODESMTJS KtlTIMEYEKI. [Aug. I9OO. 



mammal and Anomodont reptile is no more than an ordinal separa- 

 tion. The gap between these two orders is certainly smaller than 

 the gap between Iguanodon and other Ornithischian reptiles, and 

 birds. It is not, however, an approximation of the Anomodont 

 type to the mammalian type as commonly conceived which Aristo- 

 desmus exhibits, but an approximation to the points which are 

 distinctly Monotrematous, and by which the Monotremata differ 

 from all other mammalia, such as the structure of the shoulder- 

 girdle, the general form of the humerus, the stoutness of the ulna, the 

 proximal development of the fibula, and the character of the proximal 

 row of the tarsus. These structures separate Monotremes from other 

 mammals, and link them with Anomodonts. Therefore, having 

 regard only to the oviparous reproduction of the Monotreme, and 

 other reptilian conditions in the structure of its soft parts, there is 

 a sense in which all these characters must be regarded either 

 as reptilian, in view of their occurrence in the Anomodontia, or as 

 Theropsidan in forming a basis for alliance between the Anomo- 

 dontia and the Monotremata. There does not appear to be any 

 closer approximation to the higher mammals in the Monotremes 

 than is shown in Aristodesmus and other described Anomodontia, 

 except in the obliteration of sutures in the skull, the distinctive 

 form of the atlas, and the presence of marsupial bones, which have 

 not yet been recognized in Anomodontia. 



The affirmed absence of prefrontal and postfrontal bones in Ornitho- 

 rhynchus deserves better demonstration than has been given. The 

 frontal bones converge forward ; their anterior termination is flanked 

 laterally by the nasal bones ; but between the nasal and the frontal 

 there is a bone in the front of the orbit, which is above the lachrymal 

 foramen. It corresponds to the prefrontal bone. There is a large 

 separate postfrontal ossification at the back of the orbit, external to 

 the junction of the frontal and parietal. These two bones, which 

 are partly separated from the frontal by a process of the parietal 

 bone, quite exclude the frontal bone from the orbital margin. The 

 presence of prefrontal and postfrontal bones in the orbit in Mono- 

 tremata goes towards showing that their resemblances to the higher 

 mammalia are associated with reptilian divergences, which establish 

 a closer relation between Ornithorhynchus and Anomodontia, and 

 other reptiles, than was obvious. (See fig. 7, p. 643.) 



In the skull of Ornithorhynchus there is a foramen above the 

 articular surface for the lower jaw, which extends longitudinally 

 from front to back, and is narrower in some skulls than in others. 

 This may be termed the supra-articular foramen. It is stated 

 by Owen to be present in the skulls of some recent reptiles. A 

 foramen is seen in the same position in Ichthyosaurus, which lies 

 between the quadrate bone on the inner side and the quadrato- 

 jugal and supratemporal, which extend to the squamosal so as to 

 define its external side. This condition is approximated to in 

 Ornithosauria. There is a foramen above the articulation which is 

 external to the quadrate bone in many Anomodonts, 1 though it is 



1 Phil. Trans. Eoy. Soc. vol. clxxx (1889) B, pi. x, %. 4. 



