ICHTHYOTOMI. 33 



arches of the axial skeleton ; and they are often strengthened 

 by delicate dermal filaments (actinotrichia) which cluster round 

 the distal part of the supports. The dermal skeleton is insig- 

 nificant, never more than one median spine and small scattered 

 tubercles. The teeth consist of sharp cusps fixed on broad 

 bases, which overlap each other more or less and are often 

 united by well-marked facettes. 



In the typical family of Pleuracanthidae the pectoral fin 

 exhibits a fringe of cartilages on both sides of its segmented 

 axis. The much-extended dorsal fin is separated from the 

 fringe-like caudal fin. The skull is amphistylic and the mouth 

 nearly terminal. The typical and best-known genus is 



Pleuracanthus (figs. 17, 18, 22, 26). The fish is elongated and round- 

 bodied, with a straight and tapering (diphycercal) tail. Fixed upon the 

 occipital region of the cranium is a long and slender, bilaterally-sym- 

 metrical, dermal spine, which is turned backwards when not erect, and 

 does not appear to have supported a fin. The spine is hollow and is 

 armed with a double longitudinal series of denticles or barbs. The fore- 

 most few neural arches are bent forwards, but from the origin of the 

 dorsal fin backwards their direction is normal ; the right and left halves 

 of each arch remain separate. Kibs are rudimentary or absent; but in 

 the caudal region the haemal arches are as large as the corresponding 

 ueurals. The branchial arches are believed to be seven in number, though 

 there may be only five. The teeth are tricuspid, but the middle denticle 

 is comparatively minute and the name Diplodus, commonly applied to the 

 isolated fossil teeth, is thus appropriate. Very delicate dermal filaments 

 (actinotrichia) are clustered round the supporting cartilages in all the fins. 

 In the paired fins all the fringing cartilages are segmented; in the pectoral 

 fin those of the anterior (preaxial) border are much more numerous and 

 better developed than those of the hinder (postaxial) border. The dorsal 

 fin is borne by a close series, of slender cartilaginous supports, which are 

 exactly twice as numerous as the neural arches to which they are ap- 

 posed ; each support is segmented into three pieces, of which the proximal 

 element is short, the middle one very long, and the outermost again short, 

 while the latter projects into the fin-membrane. The dorsal portion of 

 the caudal fin is similarly supported, but its cartilages are equal in 

 number to the neural arches. The ventral portion of the caudal fin 

 has no supports beyond the haemal spines. The anal fin is curiously 

 subdivided into two portions, of which the supports are. crowded, par- 

 tially fused together, and thus in part branching; these exhibit more 

 numerous segments than those of the other median fins. Complete 

 skeletons of Pleuracanthus have been found only in the Coal Measures 



w. 3 



