HIMALAYAN FOSSIL SHELLS. 



Ill 



Let us now see to M^hat conclusions these genera will lead us. 



The three Chambered Univalves are all extinct genera, and the Nau- 

 tilus, the only living analogue to the Ammonite, is wanting. 



Of these, the Orthoceratite has usually been considered the oldest, 

 and characteristic of the so-called transition strata. 



The Ammonite comes next in age, and occurring sparingly in the 

 transition strata, is deposited most abundantly in the Lias, and the other 

 more ancient of the secondary strata ; then becoming more and more rare, 

 as we advance to more recent deposits, it finally disappears in the strata 

 above the chalk, that go by the name of Tertiary. 



The species we have, though apparently coinciding with one of Mr. 

 Taylor's from the Lias of Yorkshire, is not one of those which has the 

 siphuncle in a raised ridge between two furrows, which are considered as 

 characteristic of this formation. I have several times looked for such 

 among the Salagrams in the Hindoo temples, but without success. 



The Belemnite is found from the Lias to the Chalk, both inclusive. 



On the other hand, the Spiral Univalves, which increase both in num- 

 ber and variety as we approach the more recent fonnations, are with us 

 totally wanting, nor have we as yet any other indications of such formations. 



Of the bivalves, the Producta is considered as the oldest genus, and 

 is most abundant in the transition formation. The slate in which it is 

 imbedded, is probably, therefore, a transition slate— the same which is 

 the repository of the Orthoceratite. 



