NAUTILOIDEA. 



839 



slightly curved, with a large marginal siphuncular tube, formed by 

 the prolongation and conjunction of the septal necks, and provided 

 internally with one or more" funnel-shaped sheaths, which are united 

 at the top with its margin. " These sheaths apparently communi- 

 cated with one another by means of the endosiphon, which passed 

 from the initial chamber into the siphun- 

 cular cavity by means of a large foramen, 

 situated on the inner curvature of the 

 siphuncle a little above the apical point " 

 (Foord). The species of Piloceras are con- 

 fined to the summit of the Cambrian and 

 the base of the Ordovician deposits (the 

 Durness Limestone of Britain and the Cal- 

 ciferous formation and Quebec group of 

 North America). 



The genus Cyrtoceri?ia (fig. 750), of the 

 Lower Ordovician rocks (Quebec Group) of Canada. (After Billings.) 

 Canada, is perhaps allied to Piloceras. In 



this type, the shell is broadly conical and is curved, and there exists 

 on the concave side of the shell a wide marginal siphuncular tube. 



Family 3. Actinoceratid^e. — In this family the shell is straight 

 or slightly curved, and the siphuncle is of large size and is composed 



Fig. 750. — Cyrtocerina typica, 

 Ordovician (Quebec Group), 



Fig. 751. — Weathered fragment of Actin- 

 oceras Bigsbyi, of the natural size, from the 

 Ordovician rocks of Lake Huron, s, Septa ; 

 en, Endosiphon, with some of its tubuli (t). 

 (After Foord.) 



Fig. 752. — Fragment of Actinoceras Bigs- 

 byi, from the Ordovician rocks of Kentucky, 

 of the natural size, s, One of the septa ; f, 

 Foramina in the siphuncle by which the 

 tubuli thrown out by the endosiphon may 

 have communicated with the septal cham- 

 bers. (After Foord.) 



of nummuloid segments or discs, its internal cavity being more or 

 less extensively contracted by the development of "obstruction-rings." 



