MOLLUSCA — CEPHALOPODA. 
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 man 5 ’’-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 . 
