AMMONITID^:. 287 



again is the "lateral-inferior" lobe; and the lobes which 

 follow this (of a variable number) are the " auxiliary " lobes 

 (A 1 , A 2 , A 3 , A 4 ). Lastly, there is a second unpaired lobe im- 

 mediately opposite to the dorsal lobe, placed upon the con- 

 cave side of the shell, and termed the " ventral " lobe. The 

 " saddles " are similarly subdivided. Between the dorsal and 

 lateral-superior lobes comes the " dorsal saddle " (SD). Next 

 to this, between the superior-lateral and inferior-lateral lobes 

 is placed the " lateral saddle " (SL) on each side ; and this 

 is followed by a variable number of " auxiliary saddles " 

 (S 1 , S 2 , S 3 , S 4 ). 



Fig. 257. One-half of the suture of Ammonites Truellii. D, Dorsal lobe, traversed 

 by the siphuncle ; L, Lateral-superior lobe ; E, Lateral-inferior lobe ; Al, A, A 3 , A4, Aux- 

 iliary lobes; SD, Dorsal saddle ; SL, Lateral saddle ; SI, S 2 , S3, S-, Auxiliary saddles. 



The aperture of the shell in the Ammonitida is commonly 

 furnished with lateral processes of greater or less length ; and 

 in some, if not in all cases, it was further protected by a horny 

 or shelly operculum. Sometimes the operculum consists of a 

 single piece : but in other cases it is divided into two sym- 

 metrical 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 -g^oup appear in the 

 Silurian Rocks; the genus Bactrites in the Lower Silurian, 

 and Goniatites in the Upper Silurian. No other Palaeozoic 

 types of the group are known ; but with the commencement 

 of the Mesozoic period begins an era in which an enormous 

 development of the Ammonitidtz took place. The genus 

 Ceratites 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, 



