OSTEOLEPWOTI 



283 



258). In this family the endoskeleton is well ossified, and has 

 been described (Traquair [443], A. S. Woodward [505]). The 

 pectoral fin has a jointed bony axis of from three to five segments, 

 bearing each a well -developed preaxial radial; large posterior 

 processes from some of the axial segments may possibly represent 

 postaxial 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 three 

 to four axial segments bearing preaxial radials (Fig. 244). The 

 asymmetrical (pleurorachic) fins of the Rhizodonts are supposed to 

 be developed from the mesorachic (archipterygial) type (p. 106) ; 



Fio. 253. 



A, Ostcolepis macrolepifotus, Ag. ; restored. (After Traquair.) ./, anal fin ; c.f, caudal fin ; d/, 

 second dorsal fin ; op, opercular ; pc.f, pectoral fin ; p.f, pelvic fin. B, restoration of Tristicho- 

 pterus Hiatus, Eg. ; Old Red Sandstone, Caithness. (After Traquair.) a./, anal, d./, dorsal, 

 e.f, epichordal, h.f, hypochordal, p.f, pelvic, and pt.f, pectoral fin ; n.l, axial lobe of caudal fin. 



but it must be remembered that there is as yet no proof of the 

 existence of a typical mesorachic endoskeleton in the paired 

 fins of any Teleostome. 



The tail is either diphycercal or more or less heterocercal ; but 

 the upper epichordal lobe never quite disappears, and is supported 

 by dermal rays. Such tails, in which the epichordal lobe is only 

 somewhat smaller than the hypochordal, may be called hetero- 

 diphy cereal. 



Sub-Order 1. HAPLISTIA. 



This sub-order contains only the single species described by 

 Traquair under the name Tarrasius problematicus from the Lower 

 Carboniferous rocks of Glencartholm, Scotland [449, 455]. It 



