MOLLUSCA — CEPHALOPODA. 



165 



became coiled, its septa were thrown into folds, but these Gallery- 

 were generally of a relatively simple character with an 

 angular suture (Fig. 93 h), whence these forms are collectively 'I'able-cas 

 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 Permian, after which they give place to 

 the Ammonites. Among the genera here exhibited may be 

 noticed the above-mentioned Mimoceras, and Agoniatites 

 fecundus also showing the uncoiled initial portion. Most of 

 the Devonian goniatites are from Germany, but some species 

 have also been found in this country. Thus there is a 

 Tornoceras from the Middle Devonian of Devonshire, and 

 Gephyroceras intumesceiis from the Upper Devonian of that 

 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. The best known of these is 

 Clymenia, 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 Glyjphioceras (Fig. 93 V) and Brancoceras, 

 Gastrioceras with tubercles, Pericydus with transverse ribs, 

 and Prolecanites and Pronorites with many-lobecl sutures 

 (Fig 93 a). Near the last is the tiny Dimorphoceras discrcpans. 

 The goniatites are fully dealt with in Vol. III. of the 

 Catalogue of Tossil Cephalopoda (1897). 



The transition from Goniatites to Ammonites took place 

 gradually along many lines, which are being worked 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 with some 

 exactness, owing to the fact that each individual shell in its 

 early stages still possesses the structure that characterised 

 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 always preserved and 

 can often be clearly seen. We have, for example, already 

 noticed how the straight Bactritcs stage is repeated in 

 Mimoceras. It is often found that two species closely 

 resembling one another in adult stages differ so greatly in 

 their earlier stages as to lead to the conclusion that they 



