250 FiACULlTES, PHYLLOCERAS. 



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- 

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

 provided with an irregular, shaft or body, as Pictet has already 

 considered it. 



Fifty-one species. 



Genus BACUL1TES, Lamarck. 



The completely 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 superfluous, as no change 

 is here made. 



Genus PHYLLOCEBAS, Suess. 



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 Phylloceratidsi' 1 (ranch 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 variation becomes 

 :ip parent 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 

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

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

 line never tnkcs pl;i<-<>. which would seem to indicate an affinity 

 here to the crrtMcrous Ceratites, as has been thought by some 



