348 



GEOLOGICAL BIOLOGY. 



the most recent formations; on the other hand, there are known Am- 

 monites (Buchicera) from the Middle and Upper Cretaceous, the sutures 

 of which represent the Ceratite stadium (Fig. 118; also compare with Fig. 

 114) by retrocession, if they be not 

 quite the same genera. In all 

 typical Ammonites there is devel- 

 oped, besides the external ventral 

 lobe, which, in the forms with an 

 external siphuncle, is called often 

 also siphonal lobe, two main lobes 

 on the side the first and second 

 lateral lobe. Besides the external 

 lobe, there are two large external 

 saddles; and besides the lateral 

 lobes, the two primary lateral sad- 

 dles. The external is almost al- 

 ways profoundly slashed into two 

 points by the development of a 

 secondary median saddle, while 

 the internal lobe (dorsal lobe) op- 

 posite ordinarily remains entire. 

 The external saddles are also able 



to be divided sometimes by deep FlG - . Tissotia Foumeli *Rxy\e. Cenoman- 



ian, Algeria. (After Bayle.) 

 secondary indentations. In some 



genera (Pinacoceras) the differentiation of the external part of the external 

 saddle goes so far that there are intercalated between it and the external 

 lobe a greater or less number of supernumerary saddles and lobes. All 

 the saddles and all the lobes from the second lateral saddle to the internal 

 contact suture of the whorl are called external; those which are within the 

 contact sutures up to the inner saddle receive the name internal auxiliary 

 lobes and saddles. 



The variability in the number and size of the lobes is, generally, 

 in relation with the form of the shell. If the whorls are circular, 

 one observes, ordinarily, only a few lobes, and in that case they are of 

 nearly equal dimensions (Lytoceras); upon a wide ventral side the ex- 

 ternal lobe and the external saddle acquire considerable dimensions; the 

 more flat the sides are and the thinner the ventral part, the larger the 

 size of the lateral lobes and lateral saddles, and the more numerous the 

 auxiliary lobes. 



Two Divisions of the Retrosiphonatae : Goniatites and Clymenias. 

 In following the course of evolution of this group, as indi- 

 cated by the modifications of the suture-line, we begin with 

 the first division of the Ammonoidea the Retrosiplionatce of 

 Fischer. The two groups are the Goniatites and the Clyme- 

 nias. The fundamental and constant difference is found in 

 the relative position of the siphuncle. In the Goniatites the 

 siphuncle is external and in the Clymenias always internal. 



The Goniatitinse, of Hyatt's classification, begin in the 



