﻿INTRODUCTION. 



Amiada. How far certain Mesozoic genera with cycloidal scales and well-developed 

 vertebral column {Leptolepis, Oligopleurus, Megalurus, &c.) may be related to Amia 

 remains matter for additional investigation. 



b. " Incerta Sedis.'' 



Acanthodei. — Scales minute, shagreen-like ; lateral line running between two rows 

 of modified scales ; tail heterocercal ; fins furnished in front with strong spines, those of 

 the pectoral attached to a clavicle ; the other spines passing at their bases deep into the 

 flesh ; gills apparently naked ; presence of cranial roof bones doubtful. Acanthodes, 

 Cheir acanthus, &c. 



Placodermi. — Head and body enclosed in large osseous plates ; tail scaled in some, 

 naked in others. Pterichthys, Coccosteus, Asterolepis. 



Cephalaspida. — Head covered by a continuous shield ; body covered with small 

 angular plates or scales j internal skeleton, jaws, or teeth, not discovered. Cephalaspis, 

 Pteraspis, &c. 



The essential difference between the system given above and those usually given in 

 the handbooks of the present time consists in the Pala^oniscoid and Platysomid forms 

 being placed in the Acipenseroid series, and the omission of the suborder of " Lepido- 

 pteurida," proposed in 1866 by Professor Young for the reception of the Platysomids 

 and Pycnodonts. 1 As regards the latter, their position as true Ganoids has been 

 doubted by Professor Huxley, 2 and their systematic place is still most uncertain ; 

 provisionally they are, perhaps, best classified with the Lepidosteoidei. One thing at 

 least is clear, namely, that with the Platysomida they have nothing specially in common 

 but the mode of articulation of the scales. On the other hand, the affinities of the 

 Platysomida to the Palceoniscida are most evident, and the structural peculiarities of 

 both, but especially of the latter family, leave no doubt as to their close relationship with 

 Polyodon, with which therefore, to my mind at least, they should be included in a 

 common series. While the Palaoniscidce have been hitherto referred to the Lepidosteoid 

 series, the Sturiones were considered by Liitken not to be Ganoids, but to form a distinct 

 suborder of Physostomi, placed " between the Cartilaginous Pishes and the Ganoids, 

 with which last they are very closely related, but with which they cannot be united." 8 It 

 will, however, be shown in the first part of this Monograph, that the Palceoniscida form 



1 "On the Affinities of Platysomus and Allied Genera," 'Quart. Joum. Geol. Soc.,' xxii, 1866, 

 pp. 301—317. 



2 'Dec. Geol. Survey,' x (1861), p. 28, footnote. 



3 Op. cit., German edition, p. 39. Heckel had also wished to make a special " Fischtypus " out of 

 the Mullerian Chondrostei ; " Ueber die Ordnung der Chondrostei, und iiber Amia, Cyclurus, Notceus,"' 

 ' Sitzungsberichte der Wiener Akad.,' Bd. vi, 1851, p. 219. 



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