858 



TETRABRANCHIATE CEPHALOPODS. 



last volution. The suture has very numerous lobes and saddles, 



which are sometimes incised, sometimes comparatively simple, one 



or more adventitious lobes being usually 



present. An " Aptychus " is not known to 



have existed. 



The genus Pinacoceras itself includes 

 a number of Triassic " Ammonites," which 

 sometimes attain to a great size (a yard or 

 more in diameter), and are distinguished 

 by the extraordinary complexity of the 

 suture-lines. On the other hand, in the 

 genus Sageceras, though the sutures show 

 very numerous lobes and saddles, this com- 

 plexity does not exist, the saddles being 

 tongue-shaped and entire, w T hile the lobes 

 are simply indented. The species of this 

 genus are chiefly Triassic, but early types 

 appear in the Permo-Carboniferous (" Salt-Range Group ") of India. 

 Family 8. Phylloceratid^e. — In this family the shell (fig. 774) 

 is discoidal, the body-chamber occupying from one-half to three- 

 fourths of a volution. The surface is smooth or adorned with 



Fig. 774. — Side-view otPhyl- 

 loceras heterophyllutn. Ju- 

 rassic. 



Fig. 775. — Outline of Pliylloccras 

 heterofihylhim, showing the lobes 

 and saddles, d, Siphonal lobe ; l, 

 Superior-lateral lobe ; l', Inferior- 

 lateral lobe ; a a, Auxiliary lobes ; 

 v, Antisiphonal lobe ; d, External 

 saddle ; /, Lateral saddle. 



Fig. 776. — Suture-line of Phylloceras hetero- 

 fthyllum. d, Siphonal lobe; l, Superior-lateral 

 lobe ; l', Inferior-lateral lobe ; a «, Auxiliary 

 lobes ; d, External saddle ; /, Lateral saddle. 



transverse striae ; and the aperture is simple. The lobes and sad- 

 dles of the suture-line (fig. 776) diminish gradually in size in pro- 

 ceeding from the external towards the internal side of the shell ; 



