Aristodesmus rütimeyeri (Wiedersheim) / H.G. Seeley.

  • Seeley, H. G. (Harry Govier), 1839-1909.
Date:
1900
Catalogue details

Licence: In copyright

Credit: Aristodesmus rütimeyeri (Wiedersheim) / H.G. Seeley. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    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 Zguanodon 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 Ornit-ho- 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 Ornifhorhynchvs and Anomodontia, and other reptiles, than was obvious. (See fig. 7, p. 643.) In the skull of Ornithorhynehus 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. Itisstated bv 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 arliculation which is external to the quadrate bone in many Anomodonts,' though it is 1 Phil. Trans. Bov. Soc. to], clrxx (1680) B, pi. x, fig. 4.