278 MOLLUSCA. CEPHALOPODA 



once or more near the external border. Constrictions are often 

 present at intervals on the whorls. Sutures much divided ; external 

 and superior lateral lobes large. Inferior Oolite to Cretaceous. 

 Ex. P. plicatilis, Corallian. 



Peltoceras. Umbilicus large ; whorls generally quadrilateral, 

 with broad external margin, the inner whorls with numerous con- 

 tinuous ribs, most of which divide near the external margin, across 

 which they extend ; the ribs on the later whorls with two rows 

 of tubercles on the sides, one near the outer, the other near the 

 inner margin. Sutures moderately divided, with large external 

 saddle. Kellaways Rock and Oxfordian. Ex. P. athleta. 



Parkinsonia (fig. 107 A). Shell discoidal, umbilicus large ; 

 ribs nearly straight, sharp, bifurcating near the external border, but 

 interrupted at the external margin by a groove. Aperture with 

 processes from the sides. Body-chamber short. Sutures much 

 divided ; external and superior lateral lobes deep ; saddles broad. 

 Inferior Oolite. Ex. P. parkinsoni. 



Cosmoceras. Shell flattened ; umbilicus moderately large. 

 Eibs numerous, bifurcating at the middle of the sides of the shell, 

 where there is generally a row of tubercles, and ending in a tubercle 

 or spine at the external margin ; sometimes with tubercles at the 

 edge of the umbilicus. Body-chamber very short. Aperture with 

 long lateral lobes. Sutures much divided ; the external lobe much 

 shorter than the superior lateral lobe ; there are several auxiliary 

 lobes. Inferior Oolite to Lower Cretaceous. Ex. G. jason, Ox- 

 fordian. 



Hoplites. Shell flattened ; umbilicus usually small. Ribs 

 curved, bifurcating, and generally bearing a row of tubercles near 

 the external margin and another near the umbilicus or at the 

 middle of the sides ; external margin flattened or deeply grooved. 

 Sutures finely divided ; many auxiliary lobes. Cretaceous. Ex. 

 H. splendens, H. interruptus, Gault. 



Acanthoceras. Whorls thick ; umbilicus large ; ribs simple 

 or bifurcated, with rows of tubercles at the sides and margin ; 

 external margin broad with a median row of tubercles. Saddles 

 broad. Chalk. Ex. A. rhotomagense, Lower Chalk. 



