426 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. V. 



Triassic formation. Of the latter, P. Freieslebeni is a beau- 

 tiful and common ichthyolite. There are very rarely any 

 remains of the vertebral column. 1 



Semionotus. Wall-case A. [No. 2.] On the middle ledge 

 of this Case there is a slab of lias from Coburg, on which is a 

 group of fourteen specimens of a small, elegant, homocercal 

 lepidoid fish, with a forked tail, (S. Bergeri,} belonging to this 

 genus. 



' Osteolepis. There are on the upper shelves of this Case 

 (2), many examples of two other heterocercal families ; 1. The 

 Acanthodians, that were inhabitants of the Devonian and 

 Carboniferous seas, and are characterised by their almost 

 microscopic scales; of these the Diplacanthus (double- spine) 

 is a well-known genus of Cromarty and Caithness. 2. The 

 Dipterians have scales like the lepidoids, but differ from that 

 family in having two dorsal and two anal fins ; of this tribe, 

 the Osteolepis (bony-scale) is a genus peculiar to the Old Red ; 

 three species occur at Caithness and Cromarty ; there is a fine 

 series in the collection, of the Osteolepis macrolepidotus, and 

 0. intermedius, from Cromarty. 2 



Tetragonolepis ; Dapedius. Wall-case A. (2, 3.) At Lyme 

 Regis, and other productive localities of the Lias, large masses 

 of angular enamelled scales, and occasionally entire specimens 

 of the fishes to which they belonged, have for many years 

 been collected. Sir H. De la Beche first scientifically investi- 

 gated the structure of these Ichthyolites, and pointed out 

 their characters and relations. The numerous examples sub- 

 sequently brought to light establish, according to M. Agassiz, 

 two genera of homocercal ganoids. The Dapedius (of which a 

 restored figure is given in Lign. 86) is a wide, laterally com- 

 pressed fish, with a rounded head, and fins of moderate size. 

 The body rapidly contracts towards the pedicle of the tail, the 

 fin of which is large, and symmetrically lobed. The mouth is 

 furnished with several rows of small, conical teeth, crenated at 

 their summits, and has brush-teeth on the palatine bones ; 

 the jaws are short. The scales are rhomboidal, highly 

 polished, and united laterally by short processes, as in many 

 other ganoids. 



1 See " Medals of Creation," 2d edit. 



2 See Mr. Miller's "Old lied Sandstone," PI. IV. p. 72, for details. 



