AMMONOIDEA. 



353 



w 



generic types are found, such as Baculites, Hamifes, Turrilites, 

 Ptychoceras, &c. With the close of the Cretaceous period, the 

 entire group of the Ammonoidea underwent an apparently sudden 

 and all but complete extinction, no undoubted examples of this 

 division of Cephalopods being known in any unequivocal Tertiary 

 strata, with the exception of certain " Ammonites " which have been 

 found in the Eocene deposits of California. 



In the following brief summary of the families of the Ammonoidea 

 the arrangement adopted by Zittel has been adhered to : — 



Family i. Clymeniid^:. — In this family the suture-lines show 

 simple lobes and saddles (fig. 768) ; the septal " necks " are directed 

 backwards, the shell being 

 " retrosiphonate "; and the 

 siphuncle is placed on the 

 internal side of the shell. 



The only genus included 

 in this family is Clymenia 

 itself (fig. 768), in which tm r 



the shell is coiled into a 

 flat spiral, the whorls of 

 which are in contact. The 

 inner side of each whorl 

 is deeply excavated for the 

 reception of the convexity 

 of the preceding whorl, 

 and the inner coils of 

 the spire are exposed to 

 view. The body-chamber 

 is long ; the aperture has 

 a ventral sinus ; and the 

 surface is smooth or 

 marked with fine trans- 

 verse striae. The sutures 

 are simply folded or lobed, 



and the siphuncle is marginal, and is placed on the internal or 

 concave side of the shell. The septal " necks " resemble those of 

 the Pearly Nautilus in being directed backwards, and they are some- 

 times short, or at other times invaginated into one another. The 

 protoconch resembles that of the Goniatitidce in form, and is with- 

 out a cicatrix. The known species of Clymenia are confined to the 

 Upper Devonian rocks, some of the limestones of this age in Ger- 

 many being spoken of as the " Clymenienkalk," owing to their being 

 profusely charged with fossils of this genus. 



Family 2. GoNiATiTiDyE. — In this family the shell is spirally 

 coiled, with more or less embracing whorls ; the suture-lines are 



V 



Fig. 768. — Clymenia Sedgivickii. Devonian. The 

 lower figure shows the form of the suture. 



