HISTIONOTUS. 7'.) 



are the usual three large postclavicular plates (fig. 2, pd.) ornamented like the 

 opercular bones. The pectoral fin in the type specimen measures 2*5 cm. in length 

 and comprises nine or ten rays, of which the foremost is the stoutest and fringed 

 with small fulcra; the basal half of each ray is undivided, but the distal half is 

 closely articulated and more or less subdivided. The pelvic fin is smaller, with 

 very large uniserial fulcra. The dorsal fin (PI. XVII, fig. 2) comprises about 

 twenty-five well* spaced stout rays, which are closely articulated and subdivided for 

 the greater part of their length. Its foremost rays are ornamented with smooth 

 longitudinal ridges of ganoine, and its anterior border is fringed with large 

 nniserial fulcra, of which a few are basal and gradually increase in length. The 

 anal fin is very small, with about four rays. The caudal fin is clearly forked, with 

 about eight enamelled rays in each lobe, and the fulcra on the upper lobe larger 

 than those on the lower lobe. 



All the scales, except a few between the base of the pectoral fins, are completely 

 covered with smooth enamel, and most of them even in the caudal region are finely 

 pectinated at the hinder border. The total number of transverse series, counted 

 along the course of the lateral line, is about 40 ; and the number of scales in a 

 transverse series above the pelvic fins is about 12. The principal scales of the 

 flank in the abdominal region, which have often a slightly convex pectinated edge, 

 are from two to three times as deep as broad, while those of the flank in the 

 caudal region are also deeper than broad. Nearly all the scales dorsally and 

 ventrally are at least as deep as broad, and very few are destitute of posterior 

 serration or pectination, though their shape is often more or less irregular. The 

 scales immediately bordering the dorsal fin are especially peculiar in shape, 

 truncated and usually widest at their upper end, concave at their posterior 

 pectinated edge ; four small smooth-edged scales of irregular shape are separated 

 from them beneath the fulcra and foremost ray of this fin. Three or four smooth- 

 edged diamond-shaped ridge-scales, not much enlarged, occur on the caudal pedicle 

 between the dorsal and caudal fins. As shown in the type specimen, a few 

 smooth-edged scales also occur on the narrow ventral face of the abdominal region; 

 while some of the smaller scales between and in front of the base of the pectoral 

 fins bear only isolated ridges and tubercles of enamel. The lateral line traverses 

 the seventh row of scales above the ventral border in the abdominal region. 

 Externally it is only feebly marked by a ridge and notch on each scale, with an 

 occasional perforation, but on the inner face it forms a deep groove (PI. XVII, 

 fig. 5, 1.). The smaller perforations of an upper slime-canal are also seen on the 

 scales at the base of the dorsal fin. Nearly all the scales are strengthened by a 

 broad median vertical ridge on the inner face, and most are united by a broad 

 peg-and-socket articulation (PI. XVII, fig. 5). 



Horizon and Localities. — Middle Purbeck Beds : Swanage, Dorset ; Tisbury, 

 Wiltshire. 



