250 BACULITES, I'HYLLOCERAS. 



Turrilites Senequierianus^ Orb., is also to be referred there, 

 which is distinguished by its habits from all other Turrilites, 

 and approaching very closely the earlier whorls of Heteroceras^ 

 with which also, according to Pictet, it has in common the un. 

 symmetrical build of the lateral lobes. Possibly T. Senequieri- 

 anus is only the young of what in the adult state is a Heteroceras 

 provided with :ni irregular shaft or body, as Pictet has already 

 considered it. 



Fifty-one species. 



Geiiu.s BACULITES, Lamarck. 



The comi)letely straight Ammonites of the cretaceous have 

 been embraced in the genus Baculites and form a very good 

 natural group, which in the structure of the first lateral lobe is 

 allied to Lytoceras and Hamites : in fact, between a Hamites 

 with two straight limbs and a Baculites there is no important 

 difference. A list of species of Baculites and a repetition of 

 the diagnosis of the genus would be supertluous, as no change 

 is here made. 



Genus PHYLLOCERAS, Sue.'^s. 



Shell discoidal, involute, with feeble sculpture, sometimes with 

 constrictions or varices, lines of growth directed forwards ; 

 body-chamber short, margin of aperture simple with somewhat 

 produced lobes on the external side ; no aptychus ; lobes 

 numerous, diminishing regularly in size, laterals without sub- 

 division into principal paired branches ; leaves or lobes of the 

 saddles very much rounded ; antisiphonal lobe two-pointed. 



The Phylloceratidffi branch off*, according to von Mojsisovics, 

 from stems of the monophyllic Lytoceratidae of the Trias : the 

 geologically oldest forms are still distinguished by few lobes 

 and a somewhat wider umbilicus. Within the limits of certain 

 series of forms a very constant direction of A^ariation becomes 

 apparent in such a way that a steadily progressive complication 

 and increase in the number of saddle lobes or leaves takes place. 



The genus fully retains the type in the cretaceous which it 

 assumed in the Jurassic, so that a doubt as to their position can 

 never arise ; namely, a reduction and simplification of the lobular 

 line never takes place, which would seem to indicate an affinity 

 here to the cretaceous Geratites, as has been thought by some 



