A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) ... / With 7 plates and 96 textfigures.

British Museum (Natural History). Department of Geology

Date:: 1911

Credit: A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) ... / With 7 plates and 96 textfigures. Source: Wellcome Collection.

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$1G5 became coiled, it.s septa were thrown into folds, but these were generally of a relatively simple character with an angular suture (Fig. 93 h), whence these forms are collectively known as Goniatites. They are usually smooth, or with only fine lines of growth, rarely with tubercles or ribs. The goniatites are mostly of Devonian and Carboniferous age, but also occur in the I’ermian, after which they give place to the Ammonites. Among the genera here exhibited may be noticed the above-mentioned Mimoccras, and Agonialitcs fccund'us also showing the uncoiled initial portion. ]\Iost of tlie Devonian goniatites are from Germany, but some species have also been found in this country. Thus there is a Tornoccras from the Middle Devonian of Devonshire, and Gcplujrocems inlamcscens from the Upper l^evonian of tliat county. Among the Upper Devonian specimens from Germany are several aptychi. Some Devonian genera form a special group, distinguished by having the siphuncle on the inner side of the whorl. Tlie best known of these is Chjinenia, and a thin section here exhibited shows this character plainly. Greater complication is noticeable in several of the Carboniferous goniatites. Here, for instance, are the closely coiled Qlyphioccras (Fig. 93 h) and Brancoceras, Gastrioceras with tubercles, Ferici/clus with transverse ribs, and Prolccanit.es and Pronorites with man5’’-lobed sutures (Fig. 93 a). Near the last is the tiny Bimorphoccras discrcpans. The goniatites are fully dealt Avith in Vol. III. of the Catalogue of Fossil Cephalopoda (1897). The transition from Goniatites to Ammonites took place gradually along many lines, w’hich are being Avorked out by a study of the Triassic species. This study is based mainly on the complications of the suture, which are far too intricate to receive further explanation in this place. Broadly speaking, it is possible to trace lines of descent Avith some exactness, owing to the fact that each individual shell in its early stages still possesses the structure that chai'acterised its adult ancestors. This is indeed only one instance of a general principle of growth affecting most living beings; but the principle can be more easily applied in the study of these coiled shells, since the early stages are ahvays preserved and can often be clearly seen. We have, for example, already noticed how the straight Bactritcs stage is repeated in Mimoccras. It is often found tliat two species closely resembling one another in adult stages differ so greatly in their earlier stages as to lead to tlie conclusion that tliey Gallei’y VII. Table-case 4.$