160 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery forms had a larger number of paired openings and could, 

 VII. one supposes, stretch out more arms (Fig. 90). Ascoceras, 



ATall-case ^j^ich occurs in the Ordovician of North America and the 

 Silurian of Europe, especially Bohemia, had a curious life- 

 history. This may be followed in the enlarged and dia- 

 grammatically coloured model which is exhibited. Beginning 

 with a narrow tubular shell, divided by transverse septa, and 

 having a simple siphuncle near the margin (Fig. 91 F), it 

 suddenly swelled out like a Poterioceras. This gave more 



Fig. 91. — Ascoceratidse. A, upper part of shell of Ascoceras manubrium, 

 cut down the middle, showing the upward-curved septa on the left ; 

 B, C, D, large curved septa of Ascoceras fistula ; E, upper part of shell 

 of Ascoceras decipiens, with septa of ordinary type formed after the 

 deposition of the upward- curved septa ; F, the shell of the same species 

 completed, showing the simple nautiloid portion (n) ; G, H, fragments 

 of an allied form, Choanoceras. (From Foord, after Lindstrom.) 



room for the visceral cone with its contained genital^'glands, 

 and naturally changed the character of the septation (Fig. 91 

 A, F). The body now had so much room that it ceased to 

 advance to any great extent. On the contrary, as the animal 

 grew older its body contracted, and first the opening of 

 the body-chamber narrowed somewhat. Then the visceral 

 hump shrank a little at its end, but more at the side, much 

 as it did in the early stages of JSndoceras. Thus the septa 

 now produced remained close together at the apex of the 

 body-chamber, and the siphuncle between them was swollen 



