Placoganoidei

The pectoral spines (fig. 43, c) are long and slender, and consist of two principal segments, both defended by finely tuberculated ganoid plates, like those of the head and trunk. From their form, they would seem to have served to aid the fish in shuffling along the sandy bottom or bed, if left dry at low-water. The fins attached to the flexible part of the body indicate a certain power of swimming, though not with any great rapidity ; they include a small dorsal and a pair of ventrals—these latter were first observed by Sir P. Egerton. The jaws are small, and possess confluent denticles.

The type-species is the Pterich,thys Millcri ; others have been based upon proportions of the cuirass, of the pectorals, and the tail ; all are from the " old red sandstone," and the great majority have been found in the Devonian strata of Ross-shire, ness, and other Scotch localities.

Genus CEPHALASPIS (lcephale, head ; aspis, buckler).—In this genus the posterior angles of the shield-shaped helmet are produced backward in a pointed form, giving to the head the form of a " saddler's knife ;" in other respects the genus closely resembles Pterichthys.

Mr. D. Page has re cently acquired specimens of Ccphalaspis from Lan arkshire tile-stones, form ing the base of the Devo nian system, which show a dorsal fin, pectoral fins, and a large heterocercal fin, besides a well-marked capsule of the eye-ball.

Cephalaspis Murchisoni occurs in the passage beds from the Silurian to the Devonian systems.

it is the earliest known indication of a vertebrate

animal. Pteraspis Lloydii occurs in the lower "old red" of Britain.

Genus COCCOSTEUS (kokkos, berry ; osteon, bone).—If a heterocercal fin were added in outline to the restoration of the fish of this genus, given in fig. 45, a correct idea would be given of the " old red" fossil, which, in the progress of its reconstruction, has suggested so many strange notions of its nature and affinities.

The helmet and cuirass are firmly united, and there is no trace of the jointed appendages, like pectoral fins, which characterize Pteriehthys. The unprotected part of the trunk shows an ossification of the neural and hEemal spines, and of their appendages, the rays of a " dorsal" and " anal " fin ; and, by the analogy of CephalazpiA, the tail was most probably terminated by an unequal-lobed fin. The lower jaw is com posed of two rami, loosely connected at the symphysis ; so that„ when displaced, as they commonly are in crushed fossil specimens, they gave the notion of the fish being provided with laterally-working jaws, like those of the lobster. But., in reality, the jaw worked vertically upon a fixed upper jaw ; both jaws being provided with from ten to twelve teeth on each side, anchylosed to the bone.

An

under-view of the cephalothoracic buckler of Coceosteus, according to Dr. Pander's restoration, is given in fig. 46, showing the sutures of most of the cephalic plates, and the external surface of the plates of the plastron. 9, rostral plate ; 7, premedian ; 5, median; 8, pre,lateral ; 6, lateral ; i6 and 24, the suborbital bone; is, preventromedian ; behind the lozenge-shaped ventromedian, and on each side, are (22) the preventrolateral and (so) the post-ventrolateral. The same figures mark the above plates in the side view (fig. 45), with the addition of (12) the darsomedian and (4) the post-dorsomedian.

The blank space between the neural (n) and luemal

(h) spines of the fossil endoskeleton indicates the position of the soft " notochord " (c), which has been dissolved away. The cylindrical gelatinous body, so called (in Latin chorda dorsalis) pre-exists to the formation of the bony bodies of the vertebrae in all vertebrate animals ; and the development of those bodies seems never to have gone beyond this embryonal phase in any paheozoic fish ; such fishes are accordingly termed " notochords'," as retaining the notochord.

There are but two genera of existing fishes which manifest, when full grown, such a structure, associated with ossified peripheral ele ments of the vertebrae—viz., the Protopterus of certain rivers of and the Lepidosiren of certain rivers of South America. Those fishes alone would, if fossi lized, present the appearance of the vertebral column shown in fig. 45, and which characterizes all the oolitic fossil ganoid fishes (see figs.

54 and 55). This persistence in palEeozoic and most mezozoic Cephalotboracic buckler, ventral aspect, Coccoatetu decipiena (Devonian).

fishes of an embryonic vertebral character, transitory in nearly all existing fishes, significantly testifies to a principle of " pro gression." The external " ganoid " surface of the buckler-plates of Coccosteus is ornamented with small hemispherical tubercles ; whence the generic name, signifying " berry-bone." The similarity of this ornamentation to that of the plates of the buckler in some tortoises, led to the belief, when the coccos teal plates were first found, of their being evidence of the chelonian genus Trionyx in Devonian beds. Passing notions also got into print of the crustaceous affinities of Coccosteus ; whence the trivial name of the type-species decipiens, or the " deceiving " Coccosteus..