MOLLUSCA. CEPHALOPODA. 177 



Hamites. Shell bent upon itself three times, the 

 parts not in contact ; the last chamber long. Suture-line 

 with the saddles and lobes deeply incised, the superior- 

 lateral lobe and often the inferior- lateral lobe being 

 divided into two paired parts. Surface smooth or orna- 

 mented with ribs, tubercles, or spines. Lower Greensand 

 to Chalk. 



Turrilites. Shell spiral, turreted, usually sinistral, all 

 the whorls in contact. Sutures with the lateral lobes 

 symmetrically divided. Surface ornamented with ribs or 

 tubercles. Gault to Chalk. 



Baculites. Shell straight, elongate-conical ; last 

 chamber large, aperture produced ventrally. Sutures 

 with the lobes symmetrically divided. Lower Greensand 

 to Upper Chalk. 



Scaphites. Shell coiled in a plane spiral ; the whorls 

 in contact and embracing, except the last, which is 

 detached from the spiral and then recurved. Last cham- 

 ber long. Surface ornamented with simple or nodular 

 ribs. Gault to Chalk. 



Crioceras. Shell coiled in a plane spiral ; the whorls 

 not in contact. Surface ornamented with ribs and often 

 with tubercles and spines. Lower Greensand to Gault. 



Ancyloceras. Like Crioceras, but the last whorl is 

 produced in a straight line and then bent back in the 

 form of a hook. Inferior Oolite to Lower Greensand. 



ORDER. DLBRANCHIATA. 



The Dibranchiates are represented at the present day 



by the various forms of cuttle-fishes ; but they are of much 



less importance geologically than the Tetrabranchs, the 



only really common fossil genus being Belemnites. Some 



w. P. 12 



