842 



DIVISIONS OF THE CEPHALOPODA. 



vaulted chambers, convex towards the aperture, and encroaching 

 considerably upon the body-chamber. This extraordinary confor- 

 mation of the septa on the dorsal side was doubtless caused by the 

 shrinking of the animal in its shell on that side ; whereby a cavity 

 was created between the shell-wall and the mollusc, and then parti- 

 tioned off in the manner above described. The short siphuncle is 

 submarginal and near the ventral or convex border of the shell ; it 

 diminishes rapidly in size towards the body-chamber. The test is 



Fig- 755- — Ascoceras Bohemicum, Silurian, Bohemia. <z, Longitudinal section, showing the 

 septa (s s) and siphuncle (si) ; b, Ventral aspect of the cast of the shell, showing the reduction in 

 size of the body-chamber caused by the encroachment of the septa ; c, Dorsal view of the same 

 specimen ; d, Bodi'-chamber, without the air-chambers ; e, Transverse section taken in the centre 

 of the fossil ; _/j Truncated posterior extremity, showing the position of the siphuncle (si) ; g, 

 Anterior extremity, showing the simple form of the aperture {ap). The letters v and d' indicate 

 the ventral and dorsal sides respectively. (After Barrande.) 



relatively thick considering the size of the shell ; its ornaments may 

 consist of transverse lines or of annulations." 



By the researches of Lindstrom it has recently been proved that 

 Ascoceras begins in the form of an Orthoceras, having a cylindrical 

 shell, with remote, simply curved septa, and a slender tubular 

 siphuncle, and that the sac-like shell above described is only the 

 final portion of the structure. When the organism has attained its 

 full growth, and has formed the inflated Ascoceras-shell, the older, 

 Orthoceratoid portion becomes decollated or cast off. It is, there- 

 fore, a matter of extreme rarity to find a complete specimen of 

 Ascoceras, exhibiting both the older and later portions of the shell. 



