592 



MOLLUSCA 



SUB-KINGDOM VI 



period, but there are numerous localised signs of retrogression, due perhaps to un- 

 favourable surroundings. Indications of this kind occur sporadically throughout the 

 Jura and become general in the Cretaceous, leading us to infer a widespread unfavour- 

 able change in their physical surroundings, .similar but more extensive than that which 

 affected European forms during the Inferior Oolite. To the latter influence should ' 

 probably be ascribed the uncoiling observed in the Spiroceratidae. With the close of 

 the Cretaceous all species disappear for ever from the pages of geological history. 



TABLE SHOWING KANGE AND PRESUMABLE KINSHIP OF 

 AMMONOID SUB -ORDERS. 



[Professor Hyatt's revision of the Nautiloids and Ammonoids for the present work 

 terminates at this point. The classification and diagnoses are condensed from an exhaustive 

 Monograph on fossil Cephalopods, at present still in MS., which embodies the results of his 

 life-study. Reference should be made to this work, when published, for a more complete 

 account of families and genera than it has been possible to give in the preceding p;i.ir< -s. 

 TRANS.] 



Sub-Class 2. DIBRANCHIATA. 



Cephalopods with only two arborescent gills in the mantle-cavity ; provided ron wi- 

 the mouth with eight or ten arms bearing suckers or hooks, two of them (when ten in 

 all are present) being often developed into long tentacles. Funnel dosed; ink -sac 

 usually present. Shell internal, or if external, is not chambered ; in many forms 

 entirely wanting. 



