OCR Text |
Show 1905.] PRIMITIVE REPTILE PROCOLOPHON. 229 in the imperfect example which adjoins the pelvis. Its proximal end is about intermediate in form between the femur in a Chelonian and in Ornithorhynchus; for the under surface of the articular head is a wide concave pit (text-fig. 37, b), not without suggestion of the bone in Saurodesmus and the small mammal from Stonesfield and certain birds. The trochanters on each side of the articular head are much less developed than in the Monotreme, and the sub-articular pit is less conspicuous in the other specimens from Fernrocks than in the Donnybrook example, which may indicate other species. The bone can best be compared with Pareiasauria. The external or posterior trochanter is produced down the shaft as a slight ridge on the under side of the bone in one specimen. The triangular section of the shaft is not so marked as in the Donnybrook specimen, and the proximal end is more flattened on the superior or anterior surface (text-fig. 37, a). The curvature of the bone is distinctly sigmoid in length (text-fig. 37, c). Distally it both thickens and widens to the articulation, where it is flattened on the inner side, concave behind, with a pulley articulation in front. One femur is longer and another shorter than the common type. There is no living reptile to which the bone approximates. Tibia and Fibula.-The tibia is much stouter than the fibula. Its proximal end is triangular, being flattened behind, more like the tibia of a mammal than of a Dinosaur. Its wide proximal end forms the larger part of the articulation with the femur. The bone is about f of the length of the femur (text-fig. 37, b). Text-fig. 38. Humerus and adjacent bones of fore limb, from Fernrocks. The Fore Limb.-The fore limb was relatively small in the Procolophon laticeps (Phil. Trans. 1889, pi. 9). The humerus is considerably expanded at the proximal end, with a large radial crest, and manifestly twisted in the shaft, much as in Aristo-desmus and in many of the Anomodontia. But the distal end is |