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Show 1889.] OF A THERIODONT REPTILE. 575 the Monotremes, it is evident that the process marked a in Plate LV. also represents the same. Further it is manifest that the process b is the distal extremity of part of the preaxial border of the scapula, which has become twisted from the line of* the acromion towards the dorsal aspect, this being most marked in Ptychosiagum (fig. 2). This dorsal torsion of the preaxial border of the scapula is a very remarkable feature, and appears to support Professor Flower's view that the preaxial border of the Monotreme scapula represents the spine of the scapula of the higher Mammals. Thus in the scapula of Ptychosiagum the body of the bone has become to a great extent three-sided, and the surface on the inner side of the preaxial border would well represent the prescapular fossa of the higher mammals, the portion on the outer side of the same the postscapular fossa, and the somewhat rounded posterior surface (left side of figure) the subscapular fossa. If, as seems most probable, we really have in this type of scapula an indication how the reptilian scapula of the Monotremes was modified into that of the higher mammals, and the acromion is rightly identified, we shall further have to assume that the acromial process also subsequently received a dorsal torsion, so as to resume its original position at the distal extremity of the preaxial border, now converted into the spine. After this long digression it will suffice to add that the scapula of the form under consideration corresponds very closely to that described as Platypodosaurus. The humerus is represented by the somewhat imperfect distal half of that of the left side, a restored figure of which is given in Plate LV. fig. 3, on a scale of one third. This specimen shows the entepicondylar foramen underlying a bar situated in the usual position on the palmar aspect of the shaft. The radial condyle is large and well preserved, and above this there is the supinator flange on the preaxial border which serves to distinguish the humerus of the Theriodonts from that of the Dicynodonts. Unfortunately, however, this flange is imperfect, so that it cannot be determined whether there was an ectepicondylar foramen. Compared with the Anomodont humeri in the British Museum this specimen agrees very closely, both in size and contour, with the cast of the corresponding portion of a left humerus from the Permian of Russia described under the name of Brithopus \ The Russian specimen (Plate LV. fig. 4) has been a good deal damaged, and its radial condyle has been broken away ; but allowing for this imperfection the general resemblance between the two specimens is very close. Trusting to this resemblance in the distal extremities of the two humeri the proximal extremity of the African bone has been restored, partly from a Russian specimen doubtless referable to Brithopus2, and partly from the large African humerus figured in pi. xii. of Owen's * Catalogue,' as Pariasaurus, but which more probably belongs to Tapinocephalus. The present specimen appears to be distinguished 1 For synonymy, see Owen, Quart. Journ. Geol. Soc. vol. xxxii. p. 352 ct seq. 2 Vide Owen, loc. cit. |
