AMMONOIDEA. 



8 5 I 



of the shell, and known as the " internal," " dorsal," " antisiphonal," 

 or " columellar" lobe. The "saddles" are similarly subdivided and 

 distinguished. Between the ventral and superior-lateral lobes comes 

 the " external saddle " (s d). Next to this, between the superior- 

 lateral and inferior-lateral lobes, is placed the first " lateral saddle " 

 (s l), followed on each side by a variable number of " auxiliary sad- 

 dles " (s 1 , s 2 , s 3 , s 4 ). 



Associated with various forms of Ammonoids, sometimes situated 

 within the body-chamber of the shell, or at other times wholly 

 detached, are found the peculiar bodies to which the name of 

 "Aptychi" has been applied. A typical Aptychus (figs. 766, 767) 



Fig. 766. — Aptychus (Trigone t/ftes) 

 lamellosus. Jurassic. (After Wood- 

 ward.) 



Fig. 767. — Aptychus carinatits, 

 within the body-chamber of an Am- 

 monite. 



consists of two bilaterally symmetrical halves, which are somewhat 

 semicircular in shape, and are apposed to one another by their 

 straight inner margins, which are destitute of teeth. In some cases 

 the Aptychus is thin and horny, but it is usually more or less exten- 

 sively calcified, and its thickness is sometimes considerable. In the 

 thick shelly types, the principal layer of the Aptychus has a peculiar 

 cellular structure. The surface may be smooth or variously sculp- 

 tured, and the inner face is commonly marked by concentric lines 

 of growth. In the bodies to which the name of Synapty chits has been 

 given, the general structure is similar to that of Aptychus, but the 

 two halves of the structure are fused in the middle line. Bodies of 

 this type have been found in the body-chamber of species of Sca- 

 phites. In Anaptychus, again, the structure is a thin, apparently 

 horny, undivided plate, which is concentrically striated. Bodies of 

 this nature have been found in association with certain forms of 

 Ammonites and Goniatites. Some of the fossils which have been 



