FAMILY IV - HOLOPTYCHIDIE.
The type-genera of this family were first recognized and characterized by the fossil scales, under the name Holoptychius (Ag.), and by the fossil teeth, under the name Rhizodus (Ow.) They include species which have left their remains in the "old red" and the coal measures. They are nearly allied to the Ccelazanthians, having, like them, but partially ossified bones and spines, the interior of which retained their primitive gristly state, and appear hollow in the fossils. The head was defended by large externally sculptured and tuberculate ganoid plates. The teeth consist of two kinds—small serial teeth, and large laniary teeth—at long intervals ; both kinds show ing the " labyrinthic" structure at their base, which is anchylosed to the jawbone.
The generic term Rhizodus is now retained for the Holoptychians of the coal measures which have more robust and obtuse serial teeth, and longer, sharper, and more slender laniaries, exemplified by the R. Hibbertit Species of true Holoptychius-- e.g., H. giganteus (Ag.), H. nobilissimus (Ag.), occur in the old red sandstone. A noble specimen of the latter species, 2 feet 6 inches in length, discovered in the old red sandstone at Clashbinnie, near Perth, is now in the paheonto * Ow en's " Odontography," logical series of the British Museum. It is chiefly remarkable for the size and bold sculpturing of the ganoid scales (fig. 50).
Large fossil teeth, with the more complex " dendriticn disposition of the tissues, charac terize a genus (Dendrodus), most pro bably of the Holoptychian family.
The complexity is produced by numerous fissures radiating from a central mass of vasodentine, which more or less fills up the pulp-cavity of the seemingly simple conical teeth of this genus. Fig.50 a is one of these fossil teeth of the natural size—a, a transverse section ; and fig. 50 b, reduced view of a portion of the same section enlarged twenty diameters. Thus magnified, a central pulp-cavity of relatively small size, and of an irregular lobulated form, is discerned, a portion of which is shown at p ; this is immediately surrounded by transverse sections of large cylindrical vascular or pulp canals of different sizes ; and beyond these there are smaller and more numerous medullary canals, which are processes of the central pulp-cavity. In the
transverse section these processes are seen to be connected together by a net-work of smaller vascular canals belonging to a coarse osseous texture, into which the pulp has been con verted, and this structure occupies the middle half of the sec tion. All the vascular canals were filled up by the opaque matrix. From the circumference of the central net-work straight pulp-fissures radiate at pretty regular intervals to the periphery of the tooth ; most of these fissures divide once, rarely twice, in their course—the division taking place some times at their origin, in others at different distances from their terminations, and the branches diverge slightly as they pro ceed. Each of the above pulp-canals or fissures is continued from a short process of the central structure, which is nected by a concave line with the adjoining process, so that the whole periphery of the transverse section of the central Magn. section of part of Dendrodua biporciaus.
coarse reticulo-vascular body of the tooth presents a crenate outline. From each ray and its primary dichotomous divi sions short branches are sent off at brief intervals, generally at right angles with the trunk, or slightly inclined towards the periphery of the tooth. These subdivide into a few short ramifications like the branches of a shrub, and terminate in irregular and somewhat angular dilations simulating leaves, but which resolve themselves into radiating fasciculi of minute dentinal tubes. There are from fifteen to twenty-five or thirty six of these short and small lateral branches on each side of the medullary rays. The teeth of Dendrodus occur in the corn-stone beds of the " Old Red " at Scat-crag, near Elgin.