264 



MOLLUSCA. CEPHALOPODA 



Actinoceras (fig. 105). External form similar to the pre- 

 ceding ; often very large. The 

 siphuncle is large, and inflated 

 between the septa so that each 

 segment is spheroidal, and con- 

 tains in the interior a large 

 amount of calcareous deposit. In 

 the centre of the siphuncle is a 

 small tube known as the endo- 

 siphon (d), from which radiating 

 tubes (c) are given off between 

 the septa and pass to the si- 

 phuncle. Ordovician to Carboni- 

 ferous Limestone ; in England 

 chiefly Carboniferous. Ex. A. 

 giganteum, Carboniferous Lime- 

 stone. 



Fig. 105. 



Diagrammatic section 

 of a portion of the shell of 

 Actinoceras. a, septum ; b, 

 siphuncle ; c, canals from en- 

 dosiphon ; d, endosiphon. 



GrOmphoceras (fig. 106). Shell ovoid, short, straight or slightly 

 curved ; section nearly circular ; 

 body- chamber very large, aper- 

 ture contracted, T-shaped. Septa 

 close together. Siphuncle sub- 

 cylindrical or beaded, sub-central, 

 placed nearer the more convex 

 side of the shell. Surface smooth 

 or with transverse ribs or striae. 

 Silurian (perhaps also Ordovician, 

 Devonian and Carboniferous). 

 Ex. G. ellipticum, Lower Lud- 

 low. 



Fig. 106. Aperture of Gomphoceras 

 bohemicum from the Silurian. 

 (From Woodward.) Natural size. 



Phragmoceras. Similar to the last, but curved and rapidly 

 increasing in diameter, laterally compressed, section oval or 

 elliptical ; siphuncle near the inner (concave) margin. Silurian. 

 Ex. P. broderipi. 



Ascoceras. Shell a little curved ; the earlier part (which is 

 rarely found) is similar to Orthoceras, but with the septa more 

 widely separated. The later formed part is sac-like and a little 

 more convex on the outer than on the inner side ; the body-chamber 



