On associated bones of a small anomodont reptile, Keirognathus cordylus (Seeley) : showing the relative dimensions of the anterior parts of the skeleton, and structure of the fore-limb and shoulder girdle / by H.G. Seeley.

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

Licence: Public Domain Mark

Credit: On associated bones of a small anomodont reptile, Keirognathus cordylus (Seeley) : showing the relative dimensions of the anterior parts of the skeleton, and structure of the fore-limb and shoulder girdle / 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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    I have introduced the bones as having an L-shape, with one limb resting on the scapula, and the other extending transversely in front of the pre- coracoid to the angles of the transverse border of the inter-clavicle. Secondly, it is possible that there may be a foramen on the anterior border of the pre-coracoid ; but I attach no importance to the indication, because the character is absent from the other specimens. Thirdly, the sternum may have been a little more posterior in position. I have made the restora- tion by drawing one side of the girdle from the specimen, and then repeating it on the other side, to make the structure symmetrical. From the description, it is evident that this shoulder girdle belongs to an ordinal type, which is distinct from all known animals. It is intermediate in many respects between Ichthyosaurus and Ornithorhynchus, but differs from both groups in the presence or relations of the pre-coracoids, sternum, and inter-clavicle. The Ichthyo- saurs have neither pre-coracoid nor sternum, and the inter-clavicle overlaps the median union of the coracoids. In the Monotremes the pre-coracoids and coracoids extend behind the inter-clavicle, instead of meeting its lateral borders; but that may be a difference of degree rather than of plan, so that the arrangement in this, as in other respects, comes nearer to Ornithorhynchus than to Ichthyosaurus. The elongation of the inter-clavicle and size of the sternum are Lacertilian characters, and in Lizards the pre-coracoid element of cartilage does not extend behind the inter-clavicle so that the right and left elements overlap, as among Urodeles and Monotremes. Hence from such a type, the Lacertilian shoulder girdle may be derived by loss of the individuality of the pre-coracoid, while that of the Monotreme would result from transverse expansion of the pre-coracoid, shortening of the inter-clavicle, and reduced size of the sternum. On the whole, the Monotreme characters in this shoulder girdle may be regarded as the more remarkable, but they are blended with Lacertilian characters. I do not, however, think that the latter features are approximations to Lizards, so much as a generalized Heptilian inheritance which suggests the Lizard type; and that, on the whole, the Chelonian shoulder girdle may be nearer to Anomodonts than the Lizard’s. For the Chelonian stock, before it developed a plastron and lost the separation of the pre-coracoid from the scapula, may well have developed a shoulder girdle in substantial harmony with that which is here restored. The Humerus. The humerus is 4'3 centims. long. It is exposed so as to show the width of the proximal end, with the size, form, and position of its radial crest; while distally it shows the transverse thickness of the shaft. The distal extremity has the articular surface moderately convex from back to front, where the thickness of the bone is about half a centimetre. A little above the articular extremity the shaft contracts somewhat, so as to give a rounded aspect to both sides of the condylar extremity, and makes the thickness of the shaft, where it