•54 1TR. E. LYDEKKER ON THE REMAINS AND AEEINITIES form, it remains to consider its generic position. At the time when I first brought the subject of this part of my paper under the Society's notice, I was inclined provisionally to employ the term Plesiosaurus in the wide sense in which it has been used by Mr Hulke. Subsequent experience has, however, led me to the con- clusion that it is advisable to separate from that genus not only those Post-Liassic forms which I have already incidentally mentioned under the name of Cirnoliosaurus, but also the present species and certain kindred forms. I shall confine, then, the name Plesiosaurus to those Sauropterygians having a neck of considerable length, compa- ratively small heads and teeth, the cervical vertebras more or less elongated, and usually with double costal facets and firmly anchylosed arches, and the pectoral girdle with a comparatively large omosternum, Fig. 9. Part of pectoral limb of Peloneustes philarchus. (About £.) fe, humerus t, radius /, ulna t radiale i, intermedium /', ulna e. formed of two elements the scapulae being widely separated in the ventral middle line, with a small and concave ventral surface, and a very large dorsal portion extending throughout the length of the bone, and no median union between their ventral portion and the coracoids. In those forms which I propose to include in Cimoliosaurus the neck is usually greatly elongated; the head and teeth are very small the cervical vertebras are more or less elongated, with single costal facets, and often complete anchylosis of the arches and ribs with the centra while the pectoral girdle is devoid of an omoster- num, and has the ventral part of the scapulae very large and flat, and the dorsal part greatly reduced in size, the ventral plates meeting in the median line, and sending down a median bar to join the coracoids, which are produced in the middle in advance of the glenoid cavity. This genus I regard as a branch in one direction from Plesiosaurus, while another branch has culminated in Plio- saurus. In the latter branch I would place the so-called Plesio-