AMMONOIDEA. 855 



prises about three hundred species, all of which are confined to the 

 Palaeozoic rocks. The earliest types appear in the later Silurian 

 deposits, and have very simple suture-lines. In the Devonian and 

 Carboniferous rocks numerous forms of Goniatites are known ; and 

 the latest representatives of the group are found in the Permo- 

 Carboniferous beds of the Salt Range in India. In certain of the 

 Devonian and Carboniferous Goniatites the sutures become com- 

 plicated by the increase of the number of lobes and saddles, till 

 a near approach is made to the structure of the septa in the 

 Ammonites. 



Family 3. Arcestid^e. — In this family the shell is discoidal, 

 and the body-chamber is very long, extending over one or one and 

 a half whorls. The surface of the shell is smooth, or may be 

 adorned with transverse striae, ribs, or folds. The suture-lines show 

 very numerous lobes and saddles, and both of these are laterally 

 incised, thus becoming foliaceous (fig. 770). No " Aptychus " is 

 present. [In some forms a horny " Anaptychus " appears to have 

 existed.] 



The members of this, as of all the remaining families of the 

 A?n?nonoidea, are distinguished from the Clymeniidce, and Goniati- 





4t 



'- 



w* 



m 



Fig. 770. — Suture of Cyclolobus Oldhami, one of the Arcestidce. Permo-Carboniferous, India. 

 (After Waagen — copied from Zittel.) 



tidce by the greater complexity of their septa and the more ornate 

 form of the sutures resulting from this ; while the septal " necks " 

 are always turned forwards, and the shell is thus " prosiphonate." 

 Formerly all these forms were grouped under a comparatively small 

 number of genera, such as Ceratites, Ammonites, Hamites, Turri- 

 lites, B acuities, &c, distinguished principally by the mode of growth 

 and the resulting form of the shell. By far the most important of 

 these groups was the comprehensive genus Ammonites, embracing 

 the great series of forms commonly known as " Ammonites." 

 Through the researches of Hyatt, Neumayr, Mojsisovics, Waagen, 

 von Zittel, and other well-known investigators, it has now been shown 

 that the old genus Ammonites can no longer be maintained, but 

 that the types formerly included under this name admit of a natural 



