359 



Ammonites as well.* In fact, we may even believe that the 

 shells of the Ammonites were better floaters than either 

 those of Spirnla or Nautilus, for these two genera are 

 retrosiphonate, the siphonal funnels passing backward ami 

 thus giving more easy access to the water; while the shells 

 of the Ammonites were prosiphonatc. their siphonal funnels 

 bending forward like the neck of a bottle, and thus making 

 the entrance of water more difficult. This conception of the 

 planktonic wanderings of the shells of cephalopods after the 

 death of the animal furnishes a satisfactory explanation of 

 manv anomalies observed in the occurrence of these animals 

 in the geologic series. It accounts especially for the sudden 

 appearance and disappearance of the same species in widely 

 separated localities, irrespective of the character of the rock , 

 or its normal faunal contents. This wide-spread distribution 

 of these shells makes them excellent index-fossils, so that 

 even small formations can readily be correlated by their 

 species of Ammonites, even though widely separated. 



It does not follow, of course, that Ammonoid shells must 

 always be regarded as strangers which have drifted to their 

 present position. In fact, it is often easy to see that such 

 has not been the case in any particular locality, from an 

 examination of the shells themselves, as well as from 

 extraneous evidence. Thus Clarke f says that the Ammo- 

 noids of the Naples beds of Western New York " bear 



sufficient demonstration in themselves that they have lived 

 and died in these sediments." Many of the most delicate 

 shells retain their apertures unbroken, and their surface 

 ornamentation uninjured, a fact which is not consistent 

 with wave and current transportation. The presence of the 

 young in all stages of development further argues for an 

 indigenous occurrence. "On the other hand," says Clarke, 

 ''there are excellent reasons for regarding the prenuncial 

 Intumescens fauna, that of the Styliola (Styliolina) lime- 

 stone, as due to transportation from some adjoining 



* Walther, '94, p. 509 ; '97, p. 258, et seq. 

 + Naples Fauna, p. 135, et seq. 



