196 Rev. J. B. Reade on Organic Remains 



the well-known and justly valued series of remains of mol- 

 lusca. 



" The orders of fishes, according to their scaly coverings, 

 are four ; viz. 



ei 1st. Scales enamelled. 

 ec Placoid fishes, whose skin is irregularly covered with large 

 or small plates, or points of enamel, as the rays and 

 sharks* (Etym. ir\a%, a broad plate) occur recent, and nu- 

 merous in the fossil state, being found in nearly all the 

 systems of strata, though the genera are mostly peculiar 

 in each system. 

 " Ganoid fishes are regularly covered with annular thick 

 scales, composed internally of bone, and externally of 

 enamel, generally smooth and bright (Etym. ryavos, splen- 

 dour). Occur recent, but more abundantly in the fossil 

 kingdom, in which fifty extinct genera have been recog- 

 nized. 

 <e M. Agassiz appears to have ascertained that the strata be- 

 low the cretaceous rocks contain very few, if any, other fishes 

 than such as are included in these orders. 



K 2nd. Scales not enamelled. 



" Ctenoid fishes have their scales of a horny or bony sub- 

 stance, without enamel ; serrated or pectinated on the 

 free posterior margin (whence their name, from /Tret?, a 

 comb). 

 " Cycloid fishes have smooth horny or bony unenamelled 

 scales, entire at the posterior margin, with concentric 

 or other lines on the outer surface (Etym. kvkXos, a 

 circle). 

 cc To the last two orders with unenamelled scales belongs by 

 far the greater proportion of existing species of fish, which, 

 according to Cuvier, exceeded 5000, but are stated by M. 

 Agassiz to amount to 8000. On the contrary, the greater num- 



* A small shark, taken a few years ago near the island of Trinidad and 

 now in my possession, has the scales arranged over every part of the hody 

 and fins with the utmost regularity. They are somewhat oval in form, the 

 larger diameter being -g^th and the smaller —^th of an inch. Three equi- 

 distant ribs strengthen this delicate tissue, and project beyond the posterior 

 margin similarly to the ribs of the scale represented in plate viii. No. 25. 

 — J.B.R. 



