The under surface of the body between the head and trunk is defended by broad bony plates, three in number. One is median and symmetrical, of an elongate lozenge shape, with the angles rounded off ; slightly convex externally, a little produced along the middle of the anterior half into something like a low quasi-keel The outer surface is sculptured by radiating furrows, except at so much of the marginal part as is overlapped by the lateral pieces, and by the scapular arch. The lateral throat-plates are attached to the anterior half of the sides of the median one, are shaped like beetles' elytra, and converge forwards. Their centre of ossification is towards their outer and back part, from which the external ridges and grooves radiate towards the inner border.
Von compares these dermal shields to the ento and epi-sternal elements of the plastron of Chelonia ; their truer homology seems to the writer to be with the median and lateral large throat-plates or scales of Megalichthys and Sudis yiyw. The ento-sternnal element is the only endo-skeletal piece uncoinbined with a dermal ossification in most Chelonia. The epi-sternal, like the hyo- and hypo-sternals, appear to be abdo minal ribs, with superadded dermal ossifications in Ch,elortitt.
The scapula (fig. 65, s') are instructively exhibited in the very young specimen of the Archegosaurus figured in t. xiv., fig. 4, of Von Meyer's treatise. The coracoids being doubtless wholly cartilaginous at that stage, are not discernible in the specimen referred to. The upper slender end of the scapula is opposite the side of the vertebral column, about the fifth neurapophysis from the head, and it curves gently downward and forward, expanding at its humeral end. This expansion is more sudden in the fully-developed animal, giving the bone the shape of a rudder, and the direction of the scapula is changed. At least in the specimens (the great majority) in which the skeleton is seen from above, the slender dorsal end of the scapula is seen overlying, or near the hinder border of the lateral throat-plate, and it extends outward and backward to its expanded humeral end. The coracoids (sz) are repre sented by a pair of flat reniform plates, with the convex border turned forward, the concave one backward ; they seem to have overlapped the smooth margins of the posterior half of the median throat-plate. It is most probable that, as in Amphiuma, a portion of the broad coracoid remained in the cartilaginous state, and that the full reniform plate answers to the ossified part of that coracoid which it resembles in shape and relative position. The position of the slender scapula, styliform and
rib-like, as in the Perennibranchiates, is instructively shown in t. xviii., figs. 1 and 2, of M. von Meyer's treatise. The coracoids, as in Amphiunta, form the chief part of the articular cavity for the humerus.
The perennibranchiate affinities of Ardtegosaurus are shown as clearly by the scapular as by the hyoidean arch. The fore limb does not exceed half the length of the head. The humerus (53) is a short thick bone, slightly constricted at the middle, expanded and rounded at both ends, the proximal one being the largest. For some time the bone is hollow and open at each end ; when ossification finally closes the terminal apertures, it shows that the ends were connected to the coracoid and to the fore-arm by interposed ligamentous matter,—not, as in true Saurian, by a synovial joint. Of the two bones of the fore arm the ulna is a little longer and larger than the radius (54). Both bones present the simplest primitive form, gently con stricted in the middle, with the proximal ends a little concave,, the distal ones a little convex. The space between the anti brachium and the metacarpus plainly bespeaks the mass of cartilage representing, as in Amphiuma, the carpal segment (56) in Archegosaurus. No trace of a carpal bone is found save in the largest and oldest examples, in which five or six small roundish ossicles are aggregated near the ulnar side of the carpus. Four digits are present ; and considering the pollex to be, as usual, wanting, the second digit answering to the medius of pentadactyle feet, is the largest, and includes at least four phalanges (58) ; these, with the metacarpals (57), are long, slender, terminally expanded, and truncate. They obvi ously supported a longish, narrow, pointed paddle. The outer most or little finger was the shortest, and has the shortest metacarpal and first phalanx.
It is true that in lifystriosaurus the fore limbs are relatively almost as short as in Archegosaurus; and the oolitic crocodile recalls the arrest of development of the same limbs in the marsupial Potoroos ; but in Archegosaurus, not only is the small size of the fore limbs, but also their type of structure, especially that of their scapular arch, closely in accordance with that in the Perenniltranchiata, as shown in the tridactyle fore-limb of the Proteus anguinus, of which a figure is added to that of the Archegosaurus in fig. 65.