6 BRITISH FOSSIL REPTILES. 



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 hfemapophyses, 

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

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

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

 with the hyosternals and hyposternals 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\ — «?12, figs. 1 and 2, although the 

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

 piece // 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, hke the nuchal, 

 pygal, and last three neural plates {ch, py, sg, s\o, and .s-il, fig. 1). This insight into 

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

 ribs or pleurapophyses (ph — p/s, fig- 2). In the loggerhead turtle, for example, 

 the first three and the tenth (?/?i, ni2, m's, and m\o) have no corresponding pleur- 

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

 there are no rudiments of ribs answering to mi and ?«2. The marginal plates are 

 not constant in number ; the Clielone mi/das has two less than the Chelone caouanna 

 has. Some species of Trionyx {Cryptopua, 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 {Gymnojms, Dum. and Bibron), and in 

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

 Chelovia, 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 hyposternals (PI. 1, fig. 2a) 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 

 hsemal, or inferior vertebral arch (ib., fig. 14), without the interposition of any of the 

 marginal pieces, which are merely applied to the outer sides of the haemapophysis 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 cartilage-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, hyosternal, 

 hyposternal,' and ' xiphisternal,' will be used as absolute designations of the combined 

 endoskeletal and exoskeletal bones of the plastron, wdthout implying assent to the 

 hypothesis that first suggested those names to Geofi"roy St. Hilaire. 



