OF CENTRAL CANADA PART IV. 289 



simple. In Clymenia it runs along the inner margin of the shell ; 

 and along the outer margin in Goniatites, Ceratites and Ammonites 

 proper. In Clymenia, Goniatites, Ceratites and most Ammonites the 

 shell is enrolled or more or less nautiloidal in aspect, but is straight 

 in Baculites, hooked or only partially enrolled in Hamites, spiral 

 in Turrilites, and variously modified in other forms. 



A few examples of Goniatites have been cited from the Devonian 

 Rocks of Western Ontario, but these appear to be of rare occur- 

 rence, and are not well characterised. The Ammonites and other 

 representatives of the Order, although abundant in the cretaceous 

 strata of the North- West and British Columbia, are wholly unknown 

 in Palaeozoic formations, and are, therefore, never met with in the 

 rocks of Central Canada. 



Dibranchiata : This Order distinguished by the presence of two 

 branchiae, and the possession of eight or ten powerful arms or 

 prehensile organs furnished (typically) on their inner side with 

 hooked suckers is rich in living representatives. The Argonaut, 

 Octopus, Sepia or Cuttle-fish, Squid, Spirula, and the extinct 

 Belemnites, are its principal types. The female Argonaut the so- 

 called " paper Nautilus" secretes an external boat-shaped shell, 

 consisting of a single chamber. In all other forms of Dibranchiate 

 Cephalopods, there is only an internal, often horny, shell; or the 

 animal is practically without a shell. The Order is commonly sub- 

 divided into two sub-Orders : Octopoda, with eight arms, and 

 Decapoda with eight arms and two longer tentacles. In the living 

 spirula and the extinct belemnites (both belonging to the latter 

 sub-division), the internal shell is furnished with a " phragmocone" or 

 set of air-chambers divided by simply concave septa, and thus 

 resembling to some extent the external shell of the Tetrabranchiate 

 and Ammonitoidal Cephalopods. 



Remains of this sub-Order are unknown in Palaeozoic rocks ; and 

 with the exception of the belemnites, confined to Mesozoic strata, 

 they are rare in higher formations. 



19 



