A treatise on zoology . horter, and less acutely lobate ; in these also the endoskeleton is unknown. The Khizodontidae have lobate fins departing from the mesorachic type : here the preaxial edge and its dermal rays are becoming stronger than the postaxial, as in the Actinopterygii, while the skeletal axis is somewhat posterior (Fig. OSTEOLEPIDOTI 283 258). In this f;imily the endoskeletou is well ossified, and hasbeen described (Trariuair [443], A. S. Woodward [505]). Thepectoral fin has a jointed bony axis of from three to five segments,bearing each a well-developed preaxial radial; large po
A treatise on zoology . horter, and less acutely lobate ; in these also the endoskeleton is unknown. The Khizodontidae have lobate fins departing from the mesorachic type : here the preaxial edge and its dermal rays are becoming stronger than the postaxial, as in the Actinopterygii, while the skeletal axis is somewhat posterior (Fig. OSTEOLEPIDOTI 283 258). In this f;imily the endoskeletou is well ossified, and hasbeen described (Trariuair [443], A. S. Woodward [505]). Thepectoral fin has a jointed bony axis of from three to five segments,bearing each a well-developed preaxial radial; large posteriorprocesses from some of the axial segments may possibly representpostaxial radials (Fig. 252). The skeleton of the pelvic fin (Traquair [459], Goodrich [173])is less well developed, but is built on the same plan, with threeto four axial segments bearing preaxial radials (Fig. 244). Theasymmetrical (pleurorachic) fins of the Khizodonts are supposed tobe developed from the mesorachic (archipterygial) type (p. 106) ;. Fig. 2.;3. A, Osteolepis maerohpidotvs, Ag. ; restored. (AfterTraquair.) , anal fin ; , caudal tin ; ,second dorsal fin ; op, opercular; , pectoral fin ; , pelvic fin. B, restoration of Tristicho-pterus alahis, Eg. ; Old Red Sandstone, Caithness. (After Traquair.) , anal, (/./, dorsal,, epichordal, , hypocliordal, , pelvic, and p>, pectoral fin ; , axial lobe of caudal tin. but it must be remembered that there is as yet no proof of theexistence of a typical mesorachic endoskeleton in the pairedfins of any Teleostome. The tail is either diphycercal or more or less heterocercal; butthe upper epichordal lobe never quite disappears, and is supportedby dermal rays. Such tails, in Avhich the epichordal lobe is onlysomewhat smaller than the hypochordal, may be called hetero-diphycercal. Sub-Order 1. HAPLISTIA, This sub-order contains only the single species described byTraquair under the name Tarrasius lyrohlcmaticus from the Lower
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