FOSSIL FISHES.— GALLERY Xo. 6. 



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

 pose 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 

 witli 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. 



Fig. 1. — The " Lancelet," Branchiostoma (Amphioxus) lanceolatum (recent), a, Mouth ; 



e, vent; b, abdominal pore. 



Some fishes have no hard structures capable of fossilization, and 

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

 — the " Sea-squirts "and the " Lancelets " — which seem to connect 

 the Yertebrata with the subkingdoms of Invertebrata. The little 

 " Lancelet" (Branchiostoma), for example, has only a membrano- 

 cartilaginous skeleton without vertebras, ribs, or jaws (Fig. 1); 

 while the ordinary Tunicates, or " Sea- squirts," are equally 

 destitute of any but the most perishable tissues. Moreover, even 

 the lowest Fishes — the Lampreys and Hag-fishes — possess no hard 

 structures beyond the minute horny teeth (Figs. 2, 3). If, there- 



Fig. 2. 



Fig. 3. 



Fig. 2. — The "Hag-fish," Myxine anstrnlis (recent). 2a, Lower aspect of head- 



2b, A single detached tooth of Muxine. 

 Fig. 3. — Mouth of Lamprey, Petromyzon fluviatilis, showing circular arrangement of teeth. 



fore, any of these creatures existed in past geological times, they 

 would leave little or no evidence of their presence among the 

 fossils preserved in the rocks ; though it is quite possible — nay even 



