Acfinopferygii — ProiospovdyU. 



103 



Wall-case, 

 No. 7. 



The Rhaetic teeth named Scnirlcldhys belong to a very similar 

 fish. 



The Chonclrosteicl^, represented by Chondrosteus (Fig. 146) 

 from the Lower Lias of Lyme Regis, perhaps also by the 

 gigantic Gyrostens from the Upper Lias of Whitby, are inter- 

 mediate between the Palseoniscidaa and the modern sturgeons. 

 The fine specimens exhibited show that the skeleton is identical 

 with that of the sturgeons (Fig. 145), and that the jaws are 

 reduced and toothless ; but the roof of tlie skull and the develop- 

 ment of the branchiostegal rays more closely resemble the 

 corresponding parts in Pala3oniscids. 



A few dermal scn.tes identical with those of the existing Table-case 

 sturgeon, Acipenser, are shown from the English Eocene. No. 40. 

 There are also pectoral fin-spines both from the Eocene and 

 Pliocene. 



Sub-order II. — Protospondyli. 



The large majority of Mesozoic fishes are closely related to 

 the existing " bony pike " {Lepidostetis) and " bow-fin " (^Amia) 

 of North American lakes and rivers. They have the upper 

 lobe of the tail excessively abbreviated, the rays of the dorsal 

 and anal fins equal in number to their supports, and no infra- 

 clavicular plates in the pectoral arch. They are represented in 

 the Collection by a very extensive series of specimens. 



The first family is that of the Semionotidse, already repre- 

 sented by one genus of small fishes, Acentrophorus, in the 

 Upper Permian. They are stout-bodied, with a small month 

 and blunt, often powerfully crushing teeth . Semionohcs and Colo' 

 hodus are Triassic and Rhfetic ; Dapedins (Fig. 147) is Liassic ; 

 and Lepidotus (Fig. 148) ranges from the Rhfetic to the Wealden. 

 The powerfiil dentition of Lepidotus, originally named Sphcero- 

 dus, is particularly noteworthy ; the successional teeth when 

 first formed in the jaw are directed away from those they are 

 destined to replace, and gradually turn through an angle of 

 180° as they come into use. 



The Macrosemiidge are elongated fishes with small mouth, 

 obtuse teeth, and extended dorsal fin, ranging from the Rhaetic 

 to the Chalk. Fine examples of Ophiopsis and Macrosemius 

 are shown from the Lithographic Stone of Bavaria and France, 

 others of Ophiopsis and Histionotus from the Purbeck Stone of 

 Dorsetshire and Wiltshire. 



The PycnodontidjB (thick-teeth) are a remarkable family 

 of deep-bodied fishes, so-called in allusion to the powerful 

 grinding teeth (Fig. 149) which arm their forwardly-displaced 

 mouth. The rhombic scales are usually so thin, that their 

 ribbed front margin is often the only part preserved, producing 

 the appearance of a series of parallel streaks from the upper to 



Wall-cases, 

 Nos. 9 to 11. 

 Table-case, 

 Nos. 40, 41. 



Table-case, 

 No. 41. 



Wall-case, 

 No. 11, and 

 Table-cases, 

 Nos. 42, 43. 



