150 GUIDE TO THE FOSSIL INVERTEI5EATE ANIMALS. 
Gallery 
VII. 
Fio. 81. — The first-formed portions of various chambered Ccphalopod 
shells, a, b, c. Nautilus pompilins : 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, c, a curved Carboniferous nautiloid, Meloceras, 
with scar (ci). /, g, h, front, side, and upper views of protoconch of 
an ammonite, Cosmoceras. i, Protoconch and first chamber of a 
belemnite. k, Protoconch and five chambers of Bactriles, the shell 
partly removed, showing siphuncle. I, m, the first two chambers of 
an allied form from which the protoconch has di.sappcared, leaving a 
cicatrix, shown in in. n, Protoconch and first five chambers of 
Miinoccras compressum, a goniatitc. o, Protoconch and part of two 
chambers of Spiirida, 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. (Prom Foord & Crick. After Barrande, Hyatt, & Branco.) 
cated ; the edges of tlie septa round the siphuncle form a 
collar projecting towards the opening of the shell ; the 
outside of tlie shell is ornamented by ribs, folds, or tubercles, 
rafliating Irom 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 
