288 



MOLLUSCA. 



several remarkable forms, such as Turrilites, Baciilites, Hamites, 

 Scaphites, and Ptychoceras. With the close of the Cretaceous 

 period the Ammonitida disappeared altogether, and no ex- 

 ample of this large and varied family has as yet been detected 

 in the Tertiaries, or is known to exist in Recent seas. 



GENERA OF AMMONITID^E. 



The genus Goniatites (fig. 258) comprises ancient forms of 

 the Ammonitidae, 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 Car- 

 boniferous. 



The genus Bactrites comprises forms 

 quite similar to Goniatites, except that 

 the shell, instead of being rolled up, is 

 straight. The genus represents Ortho- 

 ceras. from which it differs in the 



possession of lobed septa, and in the position of the siph- 

 uncle. The known species range from the Lower Silurian to 

 the Devonian. 



The genus Ceratites (fig. 259) comprises forms which re- 



