The =urostyle= is a long rod-like bone forming the posterior unsegmented continuation of the vertebral column. It is probably equivalent to three vertebrae, the tenth, eleventh, and twelfth fused together, and to an unsegmented rod of cartilage which lies ventral to the notochord. The anterior end is expanded and bears two concave articular surfaces by means of which it articulates with the sacrum. A prominent ridge runs along the dorsal surface, but gradually diminishes when traced back. The anterior portion contains a canal which is a continuation of the neural canal. At a point not far from the anterior end, this canal communicates with the exterior by a pair of minute holes which correspond with the intervertebral foramina. B. THE SKULL[56]. The skull of the Frog consists of three principal parts:-- (1) an axial part, the =cranium proper=, which encloses the brain. To it are firmly fused (2) the =capsules of the olfactory and auditory sense organs=, (3) lastly there is the =hyoid apparatus= and the =skeleton of the jaws=. The skull is by no means so completely ossified as is the vertebral column, but in addition to the cartilage bone, there is a great development of membrane bone in connection with it. The skull has a peculiarly flattened and expanded form depending on the wide lateral separation of the jaws from the cranium. (1) THE CRANIUM PROPER or Brain case. This is an unsegmented tube, which is widest behind. It remains to a considerable extent cartilaginous, but is partly converted into cartilage bone, partly sheathed in membrane bone. Its roof is imperfect, being pierced by three holes or =fontanelles=, one large anterior fontanelle (fig. 25, A, 9), and two smaller posterior fontanelles (fig. 25, A, 10). The cartilage bones of the cranium proper are the two =exoccipitals= and the =sphenethmoid=. The =exoccipitals= (figs. 24, 25, and 26, 6) are a pair of irregular bones bounding the foramen magnum at the posterior end of the skull. They almost completely surround the foramen magnum, and bear a pair of oval convex surfaces, the =occipital condyles=, with which the first vertebra articulates. The bones generally called the exoccipitals of the frog include the =epi-otic= and =opisthotic= elements of many skulls, in addition to the exoccipitals. [Illustration FIG. 24. A DORSAL, AND B VENTRAL VIEWS OF THE CRANIUM OF A COMMON FROG (_Rana temporaria_) × 2 (after PARKER). In this and the next two figs. cartilage is dotted, cartilage bones are marked with dots and dashes, membrane bones are left white.