CEPHALOPODA. 765 
General characters of cephalopods. ] 
Much may be learned in regard to the phyletic status of genera and 
species from the ornamentation of the external surface of the shell. It has, 
for example, been demonstrated, and the fact is illustrated in the following pages 
by the species Orthoceras bilineatum, that the concentric rings or annulations which 
are found in a large number of orthocerans, are of secondary growth, the earlier 
parts of the shell being free from them; thus indicating that these annulated shells rep- 
resent a more progressed condition of development than those with smooth surfaces. 
2. The structure of the sipho. The sipho is, typically, a cylindrical tube connect- 
‘ing the air-chambers and continuous from one septum to another. Actually, 
however, in most of the primitive genera, such as Nanno, Piloceras, Cameroceras and 
Vaginoceras, it has not fully attained this condition, but is in formative and 
progressive stages. Vaginoceras, represented by Orthoceras multitubulatum and O. 
longissimum of the Black River limestone, both very rare species, is said by some 
investigators to have the place of the sipho filled 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 Pi/oceras in which we know that the siphonal 
Fig. 5.—Cameroceras burchardi, showing over- Fig. 6.—Cameroceras proteiforme ; showing the 
lapping siphonal funnels. (After Dewitz). the short siphonal funnels. 
s. septa; sf. siphonal funnels. . akg ; 
wall is developed for at least a portion of its length, as it is also in Nanno and 
Cameroceras. 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 Piloceras, Cameroceras and 
Nannothe continuous apical portion of thesipho is thickened by the extravagant depo- 
sition of testaceous matter in the cavity or between the successive siphonal sheaths. In 
