sub-class vi TELEOSTEI 93 



or less numerous. Fin fulcra absent. In the living for ins — optic nerves completely 

 decussating, conns arteriosus of the heart with only two valves, and intestine without a 

 spiral valve. 



The distinction between the bony fishes and ganoids consists parti v in the 

 dermal skeleton and partly in anatomical characters, which cannot usually be 

 verified by palaeontologists. The two sub-classes are most closely related to 

 each other, and the line of demarcation between the Amioidei and the Physo- 

 stomi is often almost obliterated. This applies specially to the scales, which 

 have already become thin and elastic in the former, and are not distinguishable 

 in any respect from those of certain Physostomi, in which a thin calcified 

 layer with bone cells happens to be developed beneath the outer smooth layer. 

 Among Teleostei the internal skeleton is often characterised by its very dense 

 structure and the sparse development of bone cells. The caudal fin, unlike 

 that of the ganoids, is usually both internally and externally homocercal. In 

 the more specialised forms the pelvic fins are sometimes displaced far forwards, 

 while the rays of the dorsal fins are sometimes articulated, sometimes spinous. 



The Teleostei are divided into the two orders of Physostomi and 

 Physoclysti. 



Order 1. PHYSOSTOMI. Miiller. 



Air bladder, when present, connected by a tube with the oesophagus. Pelvic fins 

 abdominal ; all. fin rays articulated, except the foremost rays of the pectoral and dorsal 

 fins, which are sometimes spinous. Scales, when present, usually cycloid. 



Of all the bony fishes the Physostomi approach most closely the ganoids, 

 especially the Amioidei. They probably originated from the latter in the Trias, 

 becoming specialised in their own line, and soon considerably exceeding their 

 ancestors in diversity of form. They already exhibit an important develop- 

 ment in the Upper Jurassic and Lower Cretaceous, but attain their widest 

 distribution in the Tertiary and at the present day. They live partly in the 

 sea, partly in fresh-waters. 



Family 1. Leptolepidae. Smith Woodward. 



Trunk elegantly fusiform. Head with delicate membrane bones and well-developed 

 cheek plates, more or less enamelled ; parietals meeting in middle line, flanked by large 

 squamosals ; premaxilla very small ; maxilla large, entering the gape, loosely attached 

 and with two supramaxillaries ; teeth small and conical. Opercular apparatus 

 complete. A single dorsal fin. Scales ganoid and with bony layer. Upper Lias to 

 Lower Cretaceous. 



Leptolepis, Ag. (Tharsis, Giebel), (Figs. 169, 170). Usually small fishes. 

 Dorsal fin in front of anal fin, which is not much extended. Dentary bone 

 sharply rising into a thickened obtuse elevation near its anterior end. Teeth 



k. Akad. Wiss. Wien, math.-naturw. CI. vols. XLVIIL, LYI.).—Kner and Steindachaer, Neue 

 Beitrage zur Kenntniss der fossilen Fische Oesterreichs (Denkschr. Akad. Wien, vol. XXL), 1863. 

 — Kramberger, D. G., Die eocanen Fische der Baschker Schichten (Palaeontogr. vol. XXIV.). — Die 

 jungtertiare Fischfauna Croatiens, I., II. (Beitr. Palaeont. Oesterr.-Ungarns, vols. II., III.), 1882-83. 

 —Meyer, H. v., Palaeontogr. vols. II., VI. — Sauvage, II. E., Bull. Soc. geol. France, ser. 3, vols. 

 II., III., VI., XI. — Steindachner, F., Beitrage zur Kenntniss der fossilen Fischfauna Oesterreichs, 

 I. -IV. (Sitzungsb. Akad. Wien, vols. XXXVII., XXXVIII., XL., XLVIL), 1859-63. — Wettstein, A., 

 Ueber die Fischfauna des tertiaren Glarnerschiefers (Abh. schweiz. palaeont. Ges. vol. XIII. ), 1886. 



