176 MOLLUSCA. CEPHALOPODA. 



necks directed backwards. Surface usually ornamented 

 with transverse strias. Upper Devonian. 



Goniatites. Shell discoidal ; whorls embracing, um- 

 bilicus sometimes narrow, sometimes wide. ' Last chamber 

 usually large, aperture with a ventral sinus. Sutures 

 never foliaceous, sometimes angular, sometimes rounded; 

 siphuncle small, on the external margin ; septal necks 

 usually directed backwards. Upper Silurian to Carboni- 

 ferous. 



Ceratites. Shell discoidal, ornamented with ribs or 

 tubercles ; umbilicus large ; last chamber short. Saddles 

 rounded, lobes denticulate. Trias, especially Muschelkalk. 



Ammonites. Shell coiled in a plane spiral ; the 

 whorls sometimes embracing. Sutures lobed and folia- 

 ceous. Siphuncle on the external margin. Surface smooth, 

 or ornamented with striae, ribs, tubercles or spines. The 

 external margin of the shell may be rounded or provided 

 with a ridge or keel. Lias to Chalk. 



In the last chamber of some Ammonites and a few 

 other genera, a pair of calcareous or horny plates, known 

 as the Aptychus, are occasionally found ; in shape they are 

 triangular or semicircular, and the edges where the two 

 plates are in contact are straight, the others curved. The 

 Aptychus is thought to have served as an operculum. 



The species of Ammonites are very numerous and differ 

 so much from one another that they are now regarded as 

 representing many distinct genera ; these are based mainly 

 on the length of the last chamber, on the form of the 

 sutures, and the presence or absence of an aptychus, and 

 on the aperture. But in an elementary work like the 

 present it will be convenient to retain the old genus 

 Ammonites. 



