TRISTICHOPTERUS ALATUS. 



53 



bone gives attacliinent to at least a dozen rays, finely timbriated, and 

 forming the posterior fan-like portion of the fin. The integuments 

 extended over the interspinons bones as far as the commencement 

 of the true fin-rays, thus forming the lobate base so characteristic 

 of the fins in all the Coelacanthoid fishes. The upper lobe of 

 the tail contains numerous rays, the anterior ones being short 

 and fulcral, forming a marginal fringe along the upper edge of 

 the fin. A few of the upper fin-rays are given off from a set of 

 short neurapophyses, but the terminal rays seem to abut upon the 

 vertebral axis. This is prolonged through and beyond the caudal 

 fin, and is furnished at its extremity with a few fine rays forming a 

 kind of supplemental fin projecting beyond the terminal margin of 

 the normal caudal fin. This peculiar form of tail is common to many 

 of the Cselacanthoid genera, and is most fully developed in the genus 

 Godacanthus from the Permian and Carboniferous beds. It difiers 

 from the diphycercal tail" of Prof. M'Coy, in which the upper and 

 lower rays of the true caudal fin form the terminal point. This 

 form is characteristic of the genera Diplopterus and Gyroptychius 

 belonging to the Sauroidei-dipterini. The lower lobe of the 

 tail is constructed upon a plan similar to that above described as 

 occurring in the second dorsal and anal fins, but the interapophyseal 

 osselets are more numerous. They are eight or ten in number, and 

 each carries four or five rays. These rays are stouter than those of 

 the upper lobe and are more frequently jointed and subdivided ; 

 the rays, above the upper interapophysis, forming the central area 

 of the tail, impinge upon the lower periphery of the vertebral 

 column, and decrease successively in length so as to form a vertical 

 termination to this lobe of the tail. The condition of the vertebral 

 axis in this fish forms a remarkable exception to the general law 

 hitherto applicable to the greater part (if not to all) the fishes of 

 the Old Red Sandstone, and to all the Goelacanthi hitherto described. 

 If we except the genus Dipterus (some specimens of which show 

 a tendency to ossification in the caudal region) all the Devonian 

 genera have been considered Notochordal fishes, that is to say, the 

 chorda dorscdis has persisted in its embryonic condition without, 

 any trace of segmentation. In the present subject, however, the 

 whole of the vertebral axis has left its impression distinctly on 

 the matrix in one specimen, and in the other the vertebrse of 

 the caudal region are preserved entire. There can be no doubt 

 entertained therefore that in this genus the ossification and seg- 

 mentation of the column was complete, in which respects it stands 

 alone among the contemporaneous fishes. The scales more nearly 



