150 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery 

 VII. 



Fig, 81. — The first-formed portions of various chambered Cephalopod 

 shells, a, b, c, Nautilus jpompilius : a, side view ; b, front view, the 

 apex broken to show siphuncle passing through septa ; c, apex, showing 

 scar left by protoconch. d, a straight shell of Triassic age, with a 

 plug (p) at its apex, e, a curved Carboniferous nautiloid, Meloceras, 

 with scar (ci). /, g, h, front, side, and upper views of protoconch of 

 an ammonite, Cosnioceras. i, Protoconch and first chamber of a 

 belemnite. k, Protoconch and five chambers of Bactrites, the shell 

 partly removed, showing siphuncle. I, m, the first two chambers of 

 an allied form from which the protoconch has disappeared, leaving a 

 cicatrix, shown in m. n, Protoconch and first five chambers of 

 Mwnocej-as cojnpressum, a goniatite. o, Protoconch and part of two 

 chambers of Spirtcla, the shell-wall partly broken away. In g, i, k, o, 

 the siphuncle is denoted by si. c is natural size, the rest greatly 

 enlarged. (From Foord & Crick. After Barrande, Hyatt, & Branco.) 



cated; the edges of the septa round the siphuncle form a 

 collar projecting towards the opening of the shell ; the 

 outside of the shell is ornamented by ribs, folds, or tubercles, 

 radiating from the centre of the coil. In a nautilus, on the 

 other hand, the siphuncle is near the centre of the septum; 

 the edges of the septum are but slightly curved or bent, so 



