io6 



GEOLOGICAL BIOLOGY. 



families of the other two classes, so that very much is want 

 ing to a complete record of these classes. Figures 25, 26, 27 



FIG. 



FIG. 26. 



FIG. 25. Mollusca, Lamellibranch. Venus multilamella Lmk. Tertiary. A, a right valve, 

 outer surface ; lu = lunula. /?, the same interior. C. hinge of the left valve, m' = anterior, 

 n' posterior, muscular impressions ; nib = pallial sinus ; / ligamental pit ; mz = cardinal 

 teeth. 



FIG. 26. Gastropod. A, Paludina pachystoma Sdb. Tertiary Miocene. , P. avellana Neura. 

 Plidcene. 



FIG. 27. Cephalopod. Ceratites nodosus d. Haan. Triassic. A, complete shell from the 

 side ; B, front view of the same ; inr = rim of the outer chamber ; ss, ss^, ss^ hs = saddles of 

 the sutures ; <?/, s^ sl^ hi = serrate lobes of the suture lines. 



Vertebrata. Of this branch there is scarcely an order that 

 does not develop hard parts of some kind, which might be 

 preserved in fossil condition under favorable circumstances. 

 Among the lowest orders (Lancelot, Hag-fish, Lampreys) 

 there is nothing likely to be preserved, except small teeth. 

 In the cartilaginous fishes teeth are the main parts of suf- 

 ficient hardness to resist decay and disintegration, while the 



