74 



FOSSIL FISHES. GALLERY No. 6. 



As the varied layers of sandstone, limestone, and clay, which 

 compose the greater part of the superficial crust of the earth, 

 have been accumulated as sedimentary deposits in lakes, 

 estuaries, and seas, one would naturally expect that, of the 

 Vertebrate division of animals, the remains of Fishes would 

 be most frequently met with in these formations ; and such 

 is in fact the case, although, from their fragmentary state, it 

 is not always possible to determine their precise systematic 

 position. 



h a 



FIG. 96. The " Lancelot," Eranckiosloma (Amphioxus) lanceolatum (recent), a, mouth; 

 c, vent ; b, abdominal pore. 



Some fishes have no hard structures capable of fossilizatioii, 

 and such is likewise the case with most of the lower notochordal 

 forms the " Sea-squirts " and the " Lancelets " which seem 

 to connect the Vertebrata with the subkingdoms of Invertebrata. 

 The little "Lancelet" (Branchiostoma), for example, has only a 

 membrane-cartilaginous skeleton without vertebrae, ribs, or 

 jaws (Fig. 96) ; while the ordinary Tunicates, or " Sea-squirts," 

 are equally destitute of any but the most perishable tissues. 



Marsipobranchii. 



The modern lampreys and hag-fishes also possess no hard 

 structures capable of fossilization, beyond the minute horny 

 teeth (Figs. 97, 98). Technically speaking, in fact, they are not 



FIG. 97. 



FIG. 98. 



FIG. 97. The "Hag-fish," Myxine australis (recent). 97a, Lower aspect of head. 



976, A single detached tooth of Myxine. 

 FIG. 98. Mouth of Lamprey, Petromyzonfluviatilis, showing circular arrangement of teeth. 



