A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . shown beside a glass model of the animal (Fig. 78 e). Other models of living cephalopods with similar shells are exhibited, but cannot here be discussed. It is enough to realise that in one line of descent of these forms with ensheathed shell the chambered cone, long protected by a stout covering, retained its calcareous and septate nature, while in another line it became horny, and ultimately lost its septa. There was however another line of evolution,
A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . shown beside a glass model of the animal (Fig. 78 e). Other models of living cephalopods with similar shells are exhibited, but cannot here be discussed. It is enough to realise that in one line of descent of these forms with ensheathed shell the chambered cone, long protected by a stout covering, retained its calcareous and septate nature, while in another line it became horny, and ultimately lost its septa. There was however another line of evolution, the originof which is difficult to trace, because one of its conspicuouscharacters was the complete loss of the shell. Anothercharacter was the absence of the long arms, reducing thenumber to eight. These forms therefore are called Octopoda,in opposition to the ten-armed squids and cuttle-fishes,which have been called Decapoda. Study of the earlydcA^elopment of an octopus teaches us that its ancestors musthave had a shell, and it seems probable that the loss was dueto a reduction like that which took place in the squids, but. Fig. 87.—Female Argonaut swimming from left to-right. greater in extent. A drawing of the oldest Octopod known is placed in this Case. Beside it are glass models of living species of Octopoda, among which ArgonaiUa deserves mention. As shown by the models and by the exhibited Between specimens, the well-known Arc^oiiaut shell is of a different Wall-cases ^ 1 & 2. 158 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. GalleryVII. Table-cases 2&, 2, 13 &14. Wall-case14. Wall-ease1. nature to that of the other cephalopod shells. It is confinedto the female and is secreted mainly by her arms, with whichshe enfolds her body. Their inner surfaces deposit this paper-like shell, which serves as a protection for the brood. A fewexamples have been found fossil. We pass now to the General Collection, which is dividedinto the three Orders, Nautiloidea, Ammonoide
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