284 



OSTEOLEPIDO TI 



differs from the other Osteolepidoti in the possession of continuous 

 dorsal and ventral median fins. 



Family Tarrasiidae. The dermal hones of the skull and operculum 

 appear on the whole to resemble those of the Osteolepids. The front 

 legion of the trunk is naked ; but the caudal region is covered with small 

 quadrangular scales, which scarcely overlap and have a superficial resem- 

 blance to the scales of an Acanthodian. The tail is diphicercal, and the 

 notochord was persistent, there being no centra. A series of radials, 

 more numerous than the vertebral arches, supports the median fins. The 

 pectoral fin has a small rounded basal lobe ; no pelvic fin has been found. 

 This important genus is still incompletely known ; its affinities are still 

 very uncertain. In this connection it would be most interesting to 

 ascertain the structure of the outer shiny layer which covers the dermal 

 skeleton. 



Tarrasius, Traq. ; Lower Carboniferous, Dumfriesshire. 



Sub-Order 2. RHIPIDISTIA. 

 With subdivided median fins. 



Family Holoptychiidae. The pectoral fins are acutely lobate, and 

 the pelvic fins rather less acutely lobate. The tail is heterocercal, with a 

 large lower and smaller upper lobe. There is no pineal foramen. The 



Fig. 254. 



A, Holoptychius Flemingi, Ag. ; Upper Old l!'''l Sandstone, Dura Deu ; restored. (After 

 Traquair, from A. S. Woodward.) R. restoration of (Uiiptopniiuts Kinvninll, Huxley : Devonian. 

 (After Huxley, modified.) a./, anal, d./, dorsal, e.f, epichordal, hj, hypochordal, j./, pelvic, 

 and j't.f. pectoral tin. 



