New Labyrinthodont from the Karroo beds / by H.G. Seeley.

  • Harry Govier Seeley
Date:
1907
Catalogue details

Licence: In copyright

Credit: New Labyrinthodont from the Karroo beds / 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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    The transverse sections, back and front, are vertical and clean, practically unworn, and of similar transversely oblong form. In the middle is an oblong central cavity (M,), which is less than an inch wide and less than half an inch deep. From it the sutures radiate which separate the constituent bones. When the specimen is examined from the external surface the only well-marked sutures are upon the base, so that on such evidence the jaw might be interpreted as formed of two bones. The narrow straight lateral line between the surangular and splenial on the inner side might pass for a crack in the specimen. Similarly, the line in the external groove between the elements of the dentary and infra-dentary might have been supposed to be the opposite side of the same fracture but for the transverse sections. These lateral conditions are found to be natural divisions when the broken ends of the fragment are examined. More or less serrated and angulated sutures are marked by lines of black matrix as represented in the figure (see PI. XIX, Fig. 1). They are not all followed with equal facility, and the external dentary bone is obscurely divided into superior and inferior elements by suture, for the division is less evident at the other fractured end. The com- plicated suture between the internal splint bone, which I regard as the splenial, and the two bones on which it rests is also less distinct on the posterior fracture in this specimen, where the surface is more weathered, but may be followed with a magnifying-glass. The most interesting suture is the vertical division, showing that two distinct parallel bones support the teeth in two parallel rows, of which the outer row is rather the longer The external dentary bone carries five teeth, which are close set. Their crowns are broken and show no trace of a pulp cavity. The basal attachment is four times as wide as long from front to back. The attachment of the teeth is of a pleurodont type, for the external alveolar border rises fully half an inch above the rounded internal exposures of the crowns ; and on their inner side the dentary bone rises as an oblique rounded strip from the groove in which these teeth are imbedded. Both front and back fractures pass through the bases of crowns of teeth. In the middle of the base of the tooth, on what may be the posterior fracture, the crown descends in a small short oblong portion into a shallow socket, and shows the same folded structure as the rest of the tooth. The smooth surfaces of the teeth are marked with fine parallel vertical grooves upon the enamel, which vary in depth on the several teeth. Each crown decreases in size as it ascends, by becoming narrower from within outward. The bone which abuts against the dentary above the Meckelian vacuity on the inner side of the jaw is regarded as the surangular bone, and beneath it is the angular bone. The lateral junction between these bones is hidden by the splint-like covering of the splenial. The determination of these elements rests upon the sutures shown in the transverse sections. But if there is an infra-dentary, there may also be an infra-angular ; and these elements may be upper and lower divisions of the angular bone. The upper or surangular bone is small compared with the dentary, to which it is parallel and internal. At its suture with the dentary
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