48 



PAL^EONISCUS. PLATYSOMUS. 



a kind of fish), and ambly'pterus, (from the Greek, amblus, obtuse, 

 and pteron, wing), similar to those of the coal-measures ; but they 

 are not found in any formation subsequent to that we are now con- 

 sidering. 



The palaoni'scus is found in 

 the magnesian limestone of 

 England and the kupferzchiefer 

 of Germany. The head is of a 

 somewhat singular form, espe- 

 cially with regard to the ante- 

 rior portion of the face, which 

 forms a rounded projection 

 above and before the upper jaw, 



Fig. 56. Pal&oni'scus 



occasioned by the swelling out and prolongation of some of the bones of the 

 skull. The orbit of the eye is surrounded by a series of small narrow bones, 

 and the mouth is usually large, but the teeth so exceedingly small that it is 

 rarely possible to distinguish them. The jaws, however, are powerful, and 

 more especially the lower one, which is larger than the upper. Ansted. 



The genus Platyso'mus 

 (fig. 57), (from the Greek, 

 platuSj flat, and soma, body,) 

 which is found in the same 

 strata, differs considerably 

 from the palaeoni'scus, as the 

 body is of a trapezoidal form, 

 is much raised, and nearly 

 as high as it is long, while 

 from the position of the 

 scales on the edge of the 



T-" r-r m A > back and on the belly, it ap 



Fig. V.-Platyso mus. ^ ^ ^ been jj^g 



The head is large in proportion to the size of the body, the extremity of 

 the snout forms a slightly rounded projection, the mouth is small and nar- 

 row, the jaws are armed with small but very pointed teeth, the lower jaw 

 is shorter than the upper, and broader in proportion, and the operculum (or 

 bony scale covering the gills) is narrow and much elevated. The whole 

 body is covered with large scales. 



One of the most remarkable peculiarities in the structure of this fish is, 

 ihat .ilthnugh the body is flat, short, and elevated, like that of the recent 

 flat-fish, the tail instead of being, as in the latter, much forked and equally 

 lobcd arrangements which appear, in the present state of things, to be in- 

 dispensable retains in the PI ityso'mus the hr'tcrocercal character, the upper 

 portion having the vertebral column continued into it, and being much 

 longer and more powerful than the lower portion, which rather resembles a 

 small accessory fin. Ansted. 



M. Agassiz classifies fishes according to the form of their scales. AJ1 

 those fishes with angular scales regvlarly arranged and entirely covering 

 the skin, constitute the order of Ganoidcans (from the Greek, ganos, splen- 

 dour). The order of Placoideans (from the Greek, plax, a broad plate) con. 

 tains fishes whose skin is covered irregularly with plates of enamel, often 

 of considerable dimensions, but sometimes reduced to small points, like the 

 shagreen on the skin of the shark, and the prickly tubercles of the ray. 

 The order of Clennideans (from the Greek, kteis, in the genitive ktenos, a 

 comb) is characterized by horny or bony scales, jagged like the teeth of a 



