MOLLUSCA. CEPHALOPODA 279 



Crioceras. Shell coiled in a plane spiral ; the whorls not in 

 contact. Surface ornamented with ribs which in some cases 

 bifurcate and often bear tubercles and spines. Sutures with four 

 lobes. Jurassic to Chalk. Ex. C. ellipticum, Chalk. 



Ancyloceras. Like Crioceras, but the last whorl is produced 

 in a straight line and then bent back in the form of a hook. Creta- 

 ceous. Ex. A. spinigerum, Gault. 



Scaphites. Shell coiled in a plane spiral ; the whorls in 

 contact and embracing, except the last, which is free from the spiral 

 and then recurved in the form of a hook. Body-chamber long. 

 Surface ornamented with bifurcated ribs which often bear tubercles. 

 Sutures generally much divided, with several auxiliary lobes. Upper 

 Cretaceous. Ex. S. cequalis, Lower Chalk. 



Schlcenbachia. Shell with an umbilicus ; external margin 

 broad with a smooth keel ; surface with strong ribs, which are 

 slightly curved forwards and often bear tubercles. External and 

 superior lateral saddles broad ; one auxiliary lobe. Cretaceous. 

 Ex. S. varians, Lower Chalk. 



ORDER II. DIBRANCHIA 



The Dibranchia are represented at the present day by 

 the cuttle-fishes, the squids, the calamaries, octopuses, 

 paper-nautilus, etc.; they are of much less importance 

 geologically than the Nautiloids and Ammonoids, the only 

 really common fossil forms being Belemnites and its allies. 

 Some of the modern cuttle-fishes attain a length of forty 

 feet or more. 



The Dibranchia (fig. 102) have a sac-like or elongated 

 body, and possess one pair of gills only, and one pair of 

 auricles. The number of arms is limited to eight or ten ; 

 and on the inner side — that facing the mouth — they are 

 provided with rows of sucking-discs, which sometimes 

 possess horny hooks. The jaws are not calcified, and are 



