On a Hybodont Shark ( Tristychius ) from the Calciferous Sandstone Series of Eskdale (Dumfries-Shire)

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Abstract
The Elasmobranchs of the Carboniferous Period, already known by more than isolated teeth or spines, are very different from all the Elasmobranchs of the Mesozoic and Kainozoic Eras. Some of them approach the theoretical ancestors of the sub-class in their internal anatomy and in the skeletal supports of their fins; others are peculiar in the development of bilaterally symmetrical plates in the cranial roof, or in the presence of dermal head-plates; others seem to have a dentition intermediate between that of the Selachii and the Holocephali, which have been distinct orders since Jurassic times ; and a few form an unique group in which at least the symphysial teeth do not fall out when done with, but fuse with their successors and hang in front of the mouth, either forming a complete spiral or falling away at occasional intervals. Only one specimen has been briefly noticed that seems to approach more nearly the later types of sharks, and this is now worthy of detailed study and description. The specimen in question is the anterior portion of a small shark shown of the natural size in fig. A (p. 339), and it was referred by Traquair to Tristychius , probably T. arcuatus Agassiz. It is displayed on the counterpart halves of a slab of shale from the Calciferous Sandstone Series of Eskdale (Dumfries-shire), and is now in the Royal Scottish Museum, Edinburgh. I am indebted to Dr. James Ritchie, Keeper of the Natural History Department, for the opportunity of investigating it. The

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