156 PALAEONTOLOGY. 



Family IV. — Holoptychid^. 



The type-genera of this family were first recognized and 

 characterized by the fossil scales, under the name Holojptychius 

 (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 Ccelacanthians, 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, the latter placed at long intervals ; 

 both kinds shew 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 

 !g- • m0 re robust and obtuse serial teeth, and longer, 



Scale of Holo- . . 



ptycMusnoU- sharper, and more slender laniaries, exemplified by 

 vonian), half the ■& Hibberti. Species of true Holoptychius — 

 nat. size. e ^ H. giganteus (Ag.) 9 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 palse- 

 ontological series of the British Museum. It is chiefly 

 remarkable for the size and bold sculpturing of the ganoid 

 scales (fig. 67). 



Large fossil teeth, with the more complex "dendritic'' 

 disposition of the tissues, characterize a genus (Dendrodus), 

 most probably 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. 



* Owen's Odontography, 4to, 1840, pi. 35. 



