THE CEPHALOPODA 171 



appears (Tornoceras, etc.), characterized by the appearance 

 of small external and large lateral saddles (Fig. 47, c). In 

 the Upper Devonian flourished the remarkable family 

 Clyiiieniida, in which the siphuncle is on the inner or 

 dorsal margin, instead of the peripheral margin. In the 

 same period lived the Gephyroceratida and Magnosellaridce 

 (typified respectively by Manticoceras and Tornoceras, al- 

 ready described). With the beginning of the Carboni- 

 ferous period the Devonian genera vanish, while two 

 new families appear the Glyphioceratidce in which the 

 peripheral lobe comes to be divided by a small saddle, 

 and surface-ornament first begins to be noticeable 

 (Glyphioceras, Gastrioceras) and the Prolecanitida in which 

 the number of lobes and saddles is much increased, and 

 they come to have a narrow tongue-like shape (Prolecanites, 

 Fig. 47, e) . This family attains much greater complexity of 

 suture in the Permian, especially in Medlicottia (Fig. 47, /) 

 and in Cyclolobus and Popanoceras, which seem to be tran- 

 sitional from this family to the Triassic Avcestida, The 

 type-genus of this last, Arcestes (Fig. 45, c), is notable for 

 the extreme length of body-chamber, which, in combina- 

 tion with the U-shaped whorl-section due to combined 

 depression and involution, must have resulted in a most 

 extraordinarily shaped animal. 



Along with these in the Trias are found the families 

 Cevatitida and Meekocemtida (the latter also Permian, 

 Fig. 47, g), in which the suture-line is generally " cera- 

 titic," or in some genera rather more complex. The 

 former family attains to costate and tuberculate ornament, 

 but the latter are smooth, compressed, discoidal forms. 

 In the Tmchyceratida (Fig. 45, d) the surface becomes 

 ornamented by intersecting radial and longitudinal lines, 

 with tubercles at the intersections, and the periphery is 

 more or less sulcate. The suture-line is ceratitic, but 

 with saddles notched somewhat as in Asteroceras. Another 



