Coincident with this non-ossified state of the basis of the vertebral bodies of the trunk (fig. 65, c) , is the absence of the ossified occipital condyles which characterize the skull in better developed Batrachia. The fore part of the notochord has extended into the basi-sphenoid region, and its capsule has connected it by ligament to the broad flat ossifications of expansions of the same capsule, forming the basi-occipital or basi-sphenoid plate. In fig. 65 are represented the chief modifications of the vertebra, as shown in the neck, thorax, abdomen, sacrum, and tail. The vertebra of the trunk in the fully-developed full-sized animal present the following stage of ossification :— The neurapophyses (fig. 65, n) coalesce at top to form the arch, from the summit of which was developed a compressed, sub-quadrate, moderately high spine, with the truncate or slightly convex summit expanded in the fore-and-aft direction so as to touch the contiguous spines in the back ; the spines are distinct in the tail. The sides of the base of the neural arch are thickened and extended outwards into diapophyses, having a convex articular surface for the attachment of the rib, p 1 ; the fore-part is slightly produced at each angle into a zygapophysis looking upwards and a little forwards ; the hinder part was much produced backwards, supporting two thirds of the neural spine, and each angle developed into a zygapophysis, with a surface of opposite aspects to the anterior one. In the capsule of the notochord three bony plates were developed, one on the ventral surface, and one on each side, at or near the back part of the diapophysis. These bony plates may be termed cortical parts of the centrum, in the same sense in which that term is applied to the element which is called "body of the atlas" in man and Marnm Alia, and " sub-vertebral wedge-bone" at the fore-part of the neck in Enaliosauria.
As such neural or inferior cortical elements co-exist with seemingly complete centrums in the Ich,thyosaurue, thus affording ground for deeming them essentially distinct from a true centrum, the term " hypopophysis" has been proposed for such independent inferior ossifications in and from the notochordal capsule ; and by that term may be signified the sub-notochordal plates in Archegosaurv,s, which co-exist with proper hwmapophyses (h) in the tail. In the trunk they are flat, subquadrate, oblong bodies, with the angles rounded off ; in the tail they bend upwards by the extension of the ossifi cation from the under to the side parts of the notochordal capsule ; sometimes touching the lateral cortical plates. These serve to strengthen the notochord and support the interverte bral nerve in its outward passage. The ribs (p1) are short, almost straight, expanded and flattened at the ends, round and slender at the middle. They are developed throughout the trunk and along part of the tail, co-existing there with the htemal arches, as in the The htemal arches (10, which are at first open at their base, become closed by exten sion of ossification inwards from each produced angle, con verting the notch into a foramen. This forms a wide oval, the apex being produced into a long spine ; but towards the end of the tail the spine becomes shortened, and the htemal arch reduced to a mere flattened ring.
The size of the canal for the protection of the caudal blood vessels indicates the powerful muscular actions of that part, as the produced spines from both neural and htemal arches bespeak the provision made for muscular attachments, and the vertical development of the caudal swimming organ.
The skull of the A rchegosaurus appears to have retained much of its primary cartilage internally, and ossification to have been chiefly active at the surface ; where, as in the com bined dermo-neural ossifications of the skull in the sturgeons and salamandroid fishes—e. g., Polypterus, Amia, Lepidosteus- these ossifications have started from centres more numerous than those of the true vertebral system in the skull of saurian reptiles. This gives the character of the present extinct order of Batraehia.
The skull is much flattened or depressed, triangular, with rounded angles, and the front one more or less produced according to the species ; and in some species according to the age of the individual. The base is concave ; the sides nearly straight, or slightly concave. The basi-occipital appears to have retained its primordial soft, unossified state. Of the ex-occipitals, in a distinctly ossified state, no clear view has yet been had. The super-occipital (fig. 65, 4), is represented, as in the salamandroid fishes, by a pair of flat bones, more probably developed in the epicranial membrane and integu ment than in the cartilaginous protocranium. The pair of bones external to these, and forming the prominent angles of the occipital region, represent the "par-occipitals." The lower peripheral surface of the basi-sphenoidal cartilage is ossified with a concave border towards the notochord behind, to the capsule of which it seems to have been attached. The ali sphenoids were doubtless cartilaginous, and the protocranium there unaltered, as it was apparently in the ex-occipital region. The peripheral ossifications above representing the "parietal" (7), form a pair of oblong flat bones, with the " foramen parietale " in the mid-suture. External to these, and wedged between the parietals, the super- and par-occipitals, are the pair of bones answering to the " mastoids ( 8). They give attachment externally and below to the tympanic (28), and to a subsidiary bony plate, holding the position of that development of the mastoid and squamosal, which roofs over the temporal fossa in the Chelonia : it may be termed " supra squamosal" (the bone between 8 and 27 in fig. 65). The frontal bones (11), divided by a mid-suture, like the parietals, increase in length, and are continued far in advance of the orbits. The bone (12) which occupies the position of the post frontal in Chelonia is ossified from two centres, one articulating with the mastoid ( 8), the other, which is external to it, with the supra-squamosaL This other bone may be termed the "post orbital," as proposed by Von Meyer. The post-frontal extends forward above the orbit to meet the pre-frontal, separating the frontal (ii) from the orbit, as in the sturgeon (Acipenser), Polypterus, and Lepidosteus, and also in some Chelones. The pre-frontal extends far forward, terminating in a point between the nasal ('s) and lacrymal The nasals (is), divided by the. median suture, extend to the external nostrils, their prolonga tion varying with the species and age of the individual.