74 



FOSSIL FISHES.— C4ALLERY No. 6. 



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

 conipose 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. 



Fie. 96.— The " Lancelet," Byanrhuisioma. (Amphiocas) lanceolcluni (recent), o, mouth; 

 c, 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 Vertebrata with the subkingdoras of Invertebrata. 

 The little "Lancelet" {BrayicMostoma), for example, has only a 

 membrano-cartilaginous skeleton without vertebrae, ribs, or 

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

 are equally destitute of any but the most pei'ishable 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. 



Tig. 98. 



Fig. 97.— Tlie " Haif-fish, " Mi/xine auMrtilif: (recent). 97a, Lower aspect of head. 



91h, A single detached tooth of Myxinf. 

 Fig. 98. — Mouth of Lampvey, Petvoniyzon rtuviatil/s, showing circular arrangement of teeth. 



