82 



GEOLOGY. 



selves in successive 



Fig. 358.— The internal 

 shell of a belemnite, 

 restored; the lower, 

 solid, conical portion, 

 the part most fre- 

 quently preserved, is 

 the rostrum or guard ; 

 the middle portion is 

 the phragmocone, 

 which is a diminutive 

 chambered shell with 

 septa, siphuncle, and 

 protoconch as in the 

 older tetrabranch or- 

 der; the upper part is 

 the prostracum, which 

 corresponds to the 

 " pen " of the living 

 cuttle-fishes. 



of both continents. 



generations with unusual breadth and uniformity, 

 and marked with peculiar fidelity the successive 

 stages of Jurassic marine history. At least thirty 

 faunal zones have thus been distinguished in 

 Europe, and recognized in large degree in southern 

 Asia (Cutch). 



(2) The ammonites and their predecessors, the 

 ceratites, goniatites and orthoceratites, were tetra- 

 branchs and had external shells, but there had 

 been introduced in the Trias the dibranchiate 

 form which had internal shells, if any at all, and 

 these rose to prominence in the Jurassic with ex- 

 traordinary rapidity in the form of belemnites. 

 The first known of the cuttlefishes (sepeoids) also 

 appeared at this time. The belemnites were 

 cephalopods of general cuttlefish aspect, usually 

 represented in the fossil state by their internal 

 shell or " pen," as illustrated in Fig. 358. The 

 fact that the phragmocone had the characteristic 

 features of the chambered shells of the tetra- 

 branchiates in a seemingly aborted and useless 

 form, has naturally suggested that the belemnites 

 were their descendents, but this view is not en- 

 tirely without difficulties. The belemnites rose so 

 rapidly that in the course of the period they 

 almost came to rival the ammonites, and were 

 almost as characteristic of the successive stages 

 of deposition. 



(3) The pelecypods also flourished during the 

 period, and took on a markedly modern aspect, 

 the oyster famify taking the lead, the Ostrea itself 

 being common. Among the more notable genera 

 were the thick-shelled, odd-shaped Trigonia, 

 Gryphoea, Exogyra, and Ostrea, and the smooth, 

 thin-shelled Aucella of world-wide distribution 

 (Fig. 359). Certain species of Aucella were es- 

 pecially characteristic of the northern provinces 



