840 



DIVISIONS OF THE CEPHALOPODA. 



The type of this family is the genus Actinoceras (figs. 751 and 

 752), in which the shell is elongate-conical, and often of large size. 

 The siphuncle is very large, its diameter sometimes equalling half 

 that of the shell ; and it is greatly inflated between the septa, so that it 

 comes to present a series of segments of a compressed-globular form. 

 In the centre of the main siphuncular tube is a cylindrical " endo- 

 siphon" (fig. 751, en), and the space between this and the wall of 

 the former is largely occupied by secondary calcareous deposits 

 (" obstruction -rings") developed at the "necks" of the septa. 

 " The endosiphon is provided with a distinct wall, and gives off at 

 regular intervals between the septa a number of radiating canals or 

 tubuli (fig. 751, /), which apparently penetrate the shelly covering or 

 wall of the siphuncle " (Foord). The foramina by which these tubuli 

 open externally form a ring round each segment of the siphuncle 

 (fig. 752,/), and they may perhaps have served to transmit blood- 

 vessels to the lining-membrane of the air-chambers. The genus 



Actinoceras ranges from the base of the 

 Ordovician (Upper Cambrian ?) to the 

 Carboniferous limestone, the A. gigan- 

 teum of this latter formation attaining 

 a length of from four to six feet. 



Allied to Actinoceras are the genera 

 Discosorus, Huronia, and Sactoceras, 

 all of which are confined to the Silurian 

 rocks. 



Family 4. Gomphoceratid^:. — In 

 this family the shell may be approxi- 

 mately straight or more or less exten- 

 sively curved, and the aperture of the 

 shell is contracted and T-shaped (fig. 

 745). The principal genus in this 

 family is Gomp/ioceras, with which may 

 be included the forms usually spoken 

 of as Phragmoceras. 



In the most typical forms of Gom- 

 phoceras (fig. 753) the shell is approxi- 

 mately straight, or is very slightly 

 curved, and the ventral side is always more convex than the dorsal, 

 the shell being thus " exogastric " ; while the siphuncle is situated 

 towards the convex or ventral aspect. The aperture of the shell 

 (fig. 745) is T-shaped ; and the transverse orifice, corresponding 

 with the head, may be widely expanded or may be contracted 

 in the middle. The siphuncle is commonly subcentral, the septa 

 are simply curved, and the body-chamber is of large size. The 

 name of Phragnioceras has been given to forms of Go7nphoceras in 



Fig. 753. — Gomphoceras Bohemi- 

 cmn, Silurian, Bohemia. (After 

 Barrande.) 



