METAZOA MOLLUSCA. 245 



more than one half the diameter of the shell. Within the 

 siphuncle are cones, one within another, which are much 

 more plainly seen than in Piloceras. To understand this 

 structure, one must consider the fleshy body of the animal. 

 This occupied the outermost chamber of the shell. 

 Extending backward a considerable distance from its pos- 

 terior part was a bag-like prolongation which secreted 

 shell in the form of a cone. At certain periods the body 

 advanced and while resting built another cone, thus 

 giving rise to the cone-in-cone structure seen in PI. 585. 

 Extending from the spire of the cones backward to the 

 apex of the shell was a hollow tube, the endosiphuncle, 

 also plainly seen in PI. 585. 



Diphragmoceras, Piloceras, and Endoceras represent 

 generalized forms having huge siphuncles. These be- 

 come only slightly contracted in later stages of growth. 

 In fact the generalized members of this class do not pass 

 through marked changes in external form and ornamenta- 

 tion or in internal structure ; one genus, Cyrtocerina, proba- 

 bly preserving its large siphuncle unchanged throughout 

 life. 1 



Much more is known of the young of the genus Ortho- 

 ceras than of the preceding genera. Its protoconch has 

 been observed. Figs. 1-3 of PI. 586 represent the proto- 

 conch of Orthoceras elegans Munst, from the front, side, 

 and above ; (a) protoconch, (b) shell ; figs. 4-6, the proto- 

 conch of Spyroceras (= Orthoceras) crotalum Hall. This 

 protoconch is a chamberless, septa-less shell, usually very 

 much shrunken owing to the delicate substance conchi- 

 olin of which it was made. Here we have a clew to the 

 radical form that gave rise to the Cephalopods, and proba- 

 bly to the Gastropods and Pteropods as well. 



In figs. 4-6 not only is the protoconch distinctly seen 

 but also the different substages of the nepionic stage. 



1 Hyatt, Genesis of the Arietidae, Smithsonian Contrib. to Knowl- 

 edge, XXVI, 1889, p. 39. 



