A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . siphuncle ; and g, guard. The shell-wall and septaare represented by thick black lines. of thin shelly plates, which correspond to the septa moreclearly seen in Belosepia. Another Eocene fossil that appears to be descended fromthe belemnites is Spirulirostra (Fig. 85 6). In some of thelater belemnites may already be observed a shortening of theguard and a curvature of the phragmocone, processes whichtend to reduce the unncessary length of the shell. In 156 GUIDE T
A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . siphuncle ; and g, guard. The shell-wall and septaare represented by thick black lines. of thin shelly plates, which correspond to the septa moreclearly seen in Belosepia. Another Eocene fossil that appears to be descended fromthe belemnites is Spirulirostra (Fig. 85 6). In some of thelater belemnites may already be observed a shortening of theguard and a curvature of the phragmocone, processes whichtend to reduce the unncessary length of the shell. In 156 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. Gallery Spirulirostra the apex of the chambered shell is coiled quiteable c ^o^iiid, and a part of the sheath has come to lie above it; thel rest of the sheath is greatly reduced and ends in a sharppoint. By further coiling of the shell, by reduction of thepro-ostracum, and by the final disappearance of the guard,was produced the coiled shell of Spirtda (Fig. 85 c). This isvery like a shell of the earlier coiled Ammonoidea, but thesiphuncle is on the inner side of the coil, the septal necks. a h Fig. 86,—Cephalopods with thin enclosed shell. a, Belemnoteuthisantiqua, Oxford clay, Wiltshire; showing phragmocone {ph), ink-sack (i), folds of mantle (w), one of the eyes (o), and the short arms {a)with their hooks (h); one-third natural size, b, Dorateuthis syriaca,Cretaceous, Lebanon; showing shell and ink-sack as i though theenclosing mantle were transparent; nat. size. pass backwards, and the shell is very thin. It is almostentirely enclosed by the mantle. I Meanwhile a distinct line of evolution was the beautifully preserved fossils of the Oxford Clayof Christian Malford is the specimen of Belemnoteuthisantiqua described by G. A. Mantell (Fig. 86 a). Here are tobe seen 10 short hook-bearing arms, the head with its well-developed eyes, the folds of the mantle, the fins, the ink-bag. MOLLUSCA—CEPHALOPODA. 157 and the phragmo
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