434 



HISTORICAL GEOLOGY 



Fig. 406. Ordovician gastropods : A, Hormotoma gracilis; B y Maclurea logani; 

 C, Protozvarthia cancellaia; D, Cyrtolites ornatus ; E, Lophospira bicincta; F, Trocho- 

 nema umbilicatum; G, Ophileta compacta. 



Cephalopods. — This is the most highly developed class of the 

 mollusks. All Ordovician cephalopods (Fig. 407 A-D) have shells 

 such as those possessed by the nautilus of to-day. The shell is divided 



into a number of 

 chambers by trans- 

 verse partitions, 

 called septa, through 

 which a tube, the 

 siphuncle (Fig. 407 

 A and D), extends 

 from one end to the 

 other, the animal 

 living in the body 

 chamber (Fig. 407 A) 

 at the larger end. 

 The juncture of the 

 septa with the shell 

 is called the suture, 

 and, as will be seen 

 (p. 528), the shape 

 of this line is of great 

 importance in deter- 

 mining the evolution 

 ofmanygenera. The 

 shape and size of 

 Ordovician cephalo- 

 pods varied greatly, 



Fig. 407. — Ordovician cephalopods: A, 

 Trocholitii ammonius; /i, Schrcederoceras caloni; 

 C, Oncoceras pandion; I), Orthoceras multi- 

 cameratum. I \ portion of the shell is removed 

 to show the partitions and siphuncle.) 



