On parts of the skeleton of a mammal from Triassic rocks of Klipfontein, Fraserberg, South Africa (Theriodesmus phylarchus, Seeley), illustrating the reptilian inheritance in the mammalian hand / by H.G. Seeley.

  • Seeley, H. G. (Harry Govier), 1839-1909.
Date:
1888
Catalogue details

Licence: Public Domain Mark

Credit: On parts of the skeleton of a mammal from Triassic rocks of Klipfontein, Fraserberg, South Africa (Theriodesmus phylarchus, Seeley), illustrating the reptilian inheritance in the mammalian hand / by 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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    Mammals can be regarded as making an approximation towards lower Vertebrates in the plan of the skeleton, or as throwing a doubt on the interpretation of the animal as a typical Mammal; but as evidences of inheritance, the carpus, metacarpus, and digits are singularly suggestive. I would express my thanks to Dr. Henry Woodward for permission to study and describe this remarkable fossil, and for his assistance in pre- paring the beautiful cast from the slab which was made by Mr. Hall, the mason, after the specimen had been cleared from some fragments of the original bone which adhered to the natural mould. The Humerus. In the impression from the slab the humerus is crossed by the tibia and fibula, so that only its proximal and distal parts are shown : the middle parts of the shaft have been contained in the corresponding slab. The bone is 11 ‘5 centims. long. It is twisted, so that the proximal end is turned inward at an angle of about 45°. Hence the distal end is shown in profile from the outer side, while the proximal end shows the expanded width of the bone, which is comparatively flattened, with a moderate side-to-side convexity. The part of the proximal end which is exposed has an antero- posterior extent of 4T centims. It is nearly straight on the anterior border, slightly concave on the posterior border, 3-2 centims. wide at the proximal end as preserved, and 1 '7 centim. wide at the distal limit of the exposure. The proximal contour, owing to the shallowness of the impression probably, gives but an imperfect conception of the form of the proximal end of the bone. The proximal articular surface is very imper- fectly preserved. What remains of it is convex from front to back, and 2 centims. wide. Its anterior extension is apparently compressed on the ulnar side, with the small hemispherical convexity of the articular head in the middle, somewhat thickened and extended inward, so as to rise convexly above the surface of the impression. Anterior to this boss is a saddle-shaped depression, concave in length and concave transversely, and then a small trochanteroid process rises, which extended to the border of the bone. The distal exposure of the humerus shows its thickness to have exceeded 7 millims. The fragment seen is 3T centims. long. The anterior and posterior sides are sub- parallel till the distal extremity is reflected forward in the trochlear condylar articu- lation. The inner surface of the bone is somewhat flattened, and marked by a groove which ascends from the median lateral extremity of the distal condyle, and, extending upward and backward with a slightly concave curve for 2 centims., obliquely crosses the shaft. The slight elevation of this ridge causes all the distal part of the bone posterior to it, which forms the convex posterior part, to be wider transversely than the part of the shaft in front of it. This ridge is not closely paralleled by any structure known to me in the distal end of any humerus. Its direction is the reverse of the band which extends over the epicondylar humeral foramen in many Mammals, though it coincides with the direction of the vessel which such bars cover in