. The Quarterly journal of the Geological Society of London. n some of the Generic Modifications of the Plesio-saurian Pectoral Arch. By Harry Gr. Seeley, Esq., , (Read May 13, 1874.) In not having a sternum the Plesiosauria differ from the Croco-dilia and from all the Lacertian orders of Reptiles. Serpents withlimbs being as yet undiscovered, the only true Eeptilia whichadmit of comparison with Plesiosaurs in the pectoral bones are theChelonians. And even here, at first sight, the resemblance is notso evident as to command attention; for the shapes of the plastron-bones in embryon
. The Quarterly journal of the Geological Society of London. n some of the Generic Modifications of the Plesio-saurian Pectoral Arch. By Harry Gr. Seeley, Esq., , (Read May 13, 1874.) In not having a sternum the Plesiosauria differ from the Croco-dilia and from all the Lacertian orders of Reptiles. Serpents withlimbs being as yet undiscovered, the only true Eeptilia whichadmit of comparison with Plesiosaurs in the pectoral bones are theChelonians. And even here, at first sight, the resemblance is notso evident as to command attention; for the shapes of the plastron-bones in embryonic Tortoises are more suggestive of the pectoraland pelvic girdles of Plesiosaurians than are the internal chelonianbones which support the limbs, since in Plesiosaurs these osteologicalelements are expanded shields which cover much of the abdominalsurface. When, however, the embryonic pectoral arch of such aChelonian as the Ghelone my das* (fig. 1) is critically looked at, only Pig. 1.—Pectoral Arch and Bones of Plastron of Chelone mydas.(After Parker.). c. Coracoid. fc. Precoracoid. s. Scapula. unimportant osteological modification is needed to change its cha-racters to those of a Plesiosaur. The chelonian coracoid bones (c) are rod-like; but their extensionis entirely posterior to the articulation for the humerus: the bonesapproximate somewhat posteriorly, are somewhat concave on theirouter margin, and terminate in cartilages of a shoe-shaped form,which are so extended inward that their toe-like terminations meetin the median line. Then, from the humeral articulation the twoprecoracoids (pc) extend inward towards the median line; they areinclined very slightly forward, and join either by their cartilages orintervening connective tissue. If, now, a line be drawn to join the median points of meeting of* Mr. Parker, Shoulder-girdle (Ray Society), pi. xii. fig. 1. OF THE PLESIOSAUEIAN PECTOKAX AECH. 437 the precoracoids and coracoids, and a thin film of ossified connectivetiss
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