GENERA OF AMMONITID^. 75 



tected by a horny or shelly operculum. Sometimes the 

 operculum consists of a single piece : but in other cases it is 

 divided into two symmetrical halves by a straight median 

 suture. The opercula of this latter kind were originally 

 described as separate fossils, under the name of Trigonellites. 

 As regards the general distribution in time of the Ammon- 

 itidce, the earliest-known forms of the group appear in the 

 Silurian rocks, the genus Bactrites in the Lower Silurian, 

 and Goniatites in the Upper Silurian. ISTo other Palaeozoic 

 types of the group are known, except a few recently dis- 

 covered species of Ammonites in the Carboniferous rocks of 

 India ; but with the commencement of the Mesozoic period 

 begins an era in which an enormous development of the Am- 

 monitidce took place. The genus Ccrcdites is characteristically 

 Triassic. The Jurassic rocks are chiefly distinguished by 

 species of the genus Ammonites itself, though other generic 

 types are not wanting. Lastly, in the Cretaceous rocks we 

 find, along with Ammonites proper, several remarkable forms, 

 such as Turrilites, Bacidites, Hamites, Scapliites, and Ptycho- 

 ceras. With the close of the Cretaceous period the Ammon- 

 itidce disappeared altogether, and no example of this large 

 and varied family has as yet been detected in deposits of 

 undoubted Tertiary date, or is known to exist in Eecent 

 seas. It should be added, however, that Ammonites occur 

 in certain deposits in North America which are believed 

 by some geologists to be of Lower Tertiary age. 



GeNEEA of AjNIMONITIDJS. 



The genus Gonicdites (fig. 462) comprises ancient forms 

 of the Ammonitidce, in which the shell is discoidal ; the 

 sutures are simply-lobed or angulated ; and the siphuncle 

 is dorsal. The earliest-known forms of this genus are found 

 in the Upper Silurian rocks, the last in the Trias, and the 

 most in the Carboniferous. 



The genus Bactrites comprises forms quite similar to 

 Goniatites, except that the shell, instead of being rolled up, 

 is straight. The genus rejjresents Orthoceras, from which it 

 differs in the possession of lobed septa and in the position of 



