MOLLUSCA. CEPHALOPODA 



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outer (siphonal) margin. Sutures with the lobes symmetrically 

 divided. Upper Cretaceous. Ex. B. vertebralis (=faujasi), Chalk. 



Psiloceras. Shell discoidal, umbilicus large ; whorls in- 

 creasing in size very slowly, external border rounded or with a very 

 small keel ; surface smooth or striated, occasionally with ribs. 

 Body-chamber large. Sutures not much divided. Lower Lias. 

 Ex. P. planorbis. 



Arietites. Shell discoidal, umbilicus large ; whorls numerous, 

 only slightly embracing, with the external border flattened and 

 provided with a keel having a groove on each side of it. Surface 

 with strong simple ribs, which are straight or bent near the margin, 

 and may bear tubercles. Body -chamber occupying from one to one 

 and a quarter whorls. Sutures much divided, with two lateral 

 lobes and one auxiliary lobe. Lower Lias. Ex. A. bisulcatus. 



Schlotheimia. Shell flat, discoidal, umbilicus usually large. 

 Kibs strong, curved, often bifurcated in the adult, bending forward 

 at the external margin, where they meet at an angle, but are often 

 interrupted by a slight furrow or 

 smooth band at the margin. Su- 

 tures deeply divided ; superior 

 lateral lobe generally deeper than 

 the external lobe ; three or four 

 auxiliary lobes. Lower Lias. Ex. 

 S. angulata. 



iEgoceras (fig. 116). Shell 

 discoidal, with a large umbilicus ; 

 whorls rounded, without a keel, 

 ornamented with simple ribs which 

 are continuous over the external 

 margin. Lobes much divided ; 

 superior lateral lobe larger than the others. Lias. Ex. 2E. capri- 

 cornus. 



Fig. 116. JEgoceras capricornus, 

 from the Middle Lias, x \. 



Oxyn otic eras. Shell much flattened ; umbilicus small ; ex- 

 ternal margin sharp or keeled ; surface smooth or striated. Sutures 

 not deeply divided ; external saddle large, divided ; auxiliary lobes 

 present. Lias to Inferior Oolite. Ex. 0. oxynotum, Lower Lias. 



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