6 FOSSIL REPTILIA OF THE LONDON CLAY. 



Such a comparison, in my opinion, guides us to a truer view of the homologies 

 of the thoracic-abdominal bony case of the Chelonians, especially with regard to the 

 lateral or parial pieces of the plastron, than the comparison exclusively relied on by 

 Geoffroy St. Hilaire. The Plesiosaurus, by its long and flexible neck, small head, 

 expanded coracoid and pubis, and flattened bones of the paddles, comes much nearer 

 to the turtle than the crocodile does ; and its abdominal ribs, or hsemapophyses, 

 are more developed than in the crocodiles ; a comparison of the ventral surface of 

 the skeleton, such as that figured by Dr. Bucklancl, in his ' Bridgewater Treatise,' 

 vol. ii, pi. 18, fig. 3, will show how clearly those abdominal ribs would correspond 

 with the hyosternals and hyposteimals of the turtle, if they had coalesced together at 

 their middle parts, leaving their outer and inner extremities free. 



With regard to the marginal pieces m\ — mil, figs. 1 and 2, although the 

 comparisons illustrated by figs. 4, 5, 6, show that they answer rather to the intercalated 

 piece h' in the crocodile than to the entire sternal rib h in the bird ; yet the phenomena 

 of their development demonstrate that they are exclusively bones of the dermal skeleton, 

 retaining their freedom from anchylosis with the endoskeletal elements, like the nuchal, 

 pygal, and last three neural plates (eh, py, 89, sio, and su, fig. 1). This insight into 

 their true nature teaches why they do not correspond in number with the vertebral 

 ribs or pleurapoplryses (ph — pis, fig. 2). In the loggerhead turtle, for example, 

 the first three and the tenth (mi, m-2, im, and m\o) have no corresponding pleur- 

 apophyses articulating with them ; and if even ci be supposed to correspond to m3, 

 there are no rudiments of ribs answering to mi and vi2. The marginal plates are 

 not constant in number ; the Chelone mydas has two less than the Chelone caouanna 

 has. Some species of Trionyx (Cryptqpus, Dum. and Bibron) have a greater number, 

 but of smaller and less regular size, confined to the posterior part of the limb of 

 the carapace ; in other species of Trionyx (Gymnqptts, Dum. and Bibron), and in 

 Sjjharyis, the marginal part of the carapace retains its embryonic condition in all 

 Chelonia, as a stratum of cartilaginous cells in the substance of the derm, forming 

 the thickened, flexible border of the carapace. 



The rudiments of the hyosternals and lryposternals have originally the form of 

 sternal or abdominal ribs ; extend transversely, and rise at their outer extremities to 

 join those of the first and sixth pair of vertebral ribs, completing the haemal, or inferior 

 vertebral arch, without the interposition of any of the marginal pieces, which are merely 

 applied to the outer sides of the hsemapophysis or sternal ribs. The expansion 

 of the parts of the plastron, especially in the fresh-water and land tortoises, is due 

 chiefly to the ossification of a layer of cartdage-cells in the substance of the derm, 

 which ossified plates are connate with the more internal elements of the plastron, 

 representing the sternum and sternal ribs. In the following descriptions of the fossil 

 Chelonia, the terms ' entosternal, episternal, hyostemal, hyposternal,' and ' xiphisternal,' 

 will be used as absolute designations of the combined endoskeletal and exoskeletal 

 bones of the plastron, without implying assent to the hypothesis that first suggested 

 those names to Geoffroy St. Hilaire. 



