7.;:, 





Min-li may In- lt>arne<l in regard to the phyU'tic -tat us of genera and 



-pc.-i>> friuii the onianitMitatiuii i>f the external Mirface of the shell. It has, 



nplt>. IMMMI ilcnii'ii-trat.'.l. .iii.l the fact is illustrated in the following page* 



l'\ the species Orlhwnix lii/inenlum, that the concentric rin^- nr annulations which 



are fmm-l in .1 larp> iium!>er of orthocerans, are of secondary growth, the earlier 



-of the shell heingfree from them; thus indicating that these annulated shells rep- 

 resent a more programed condition of development than those with smooth -m f 



_. rif structure of the sipho. The sipho is, typically, a cylindrical tul>e connect- 



ing the air-chambers and continuous from one septum to another. Actually, 



liuwi'MT. in most of the primitive genera, such as Nanno, Pilocerns, Camerocerns ami 



it has not fully attained this condition. hut is in formative and 



longivrimum of the Black River limestone, both very rare species, is said by some 

 investigators to have the place of the sipho tilled by successive sheaths which are 

 posterior continuations of the successive septa; these forms, hence, having no true 

 sipho. This interpretation requires verification. Others have regarded these 

 shells as having a distinct siphonal wall and the sheaths as confined to the siphonal 

 cavity and occurring at intervals which have no direct connexion with the septa. 

 If the former view be correct then Vaginoceras must be regarded as representing a 

 highly elementary condition of development, but the latter interpretation of the 

 structure renders it homologous with Piloceras in which we know that the siphonal 



. b.-Cameroetnu Imrchor.li, *h..wii.i: Ki. *.-Ca*uroeera proMforme ; howlnu U.. 



,al funnels. (After Dew I U). Hi-' -h<Tt siphonal funnel*. 



-pur. >f. siphonal funnels. 



wall i>- .leveloped for at least a portion of its length, as it is also in Nanno and 

 Camtrocera*. In the two genera last named the median and later portions of the 

 sipho are constituted of deflected portions of the septa known as the siphonal 

 funnels. These often extend from one septum to, or beyond the one preceding it, 

 thus separating the siphonal cavity from that of the air-chambers; but not infre- 

 quently these funnels do not completely cross the air-chambers. In either case 

 these funnels form a discontinuous siphonal wall. In I'ilocerM, Catneroceras and 

 .V.i nno the continuous apical portion of the sipho is thickened by the extravagant depo- 

 sition of testaceous matter in the cavity or between the successive siphonal sheaths. I n 



