MANUAL OF NATURAL HISTORY. 571 



In the Molluscous Sub-Kingdom the genera of 

 the Cephalopodous family Ammonitidce were very 

 numerous in the Chalk ; among which may be no- 

 ticed the straight, chambered Baculites ; the hook- 

 shaped Hamites, the discoidal, open-whorled Crio- 

 ceras, the Ammonites, with the whorls discoid and 

 united, the obliquely- whorled Toxoceras ; the Sea- 

 phites with the last whorl detached, the Ptychoce- 

 ras bent upon itself, the twisted Anciloceras, and the 

 sinistral, turrited, spiral, chambered shell, the Tur- 

 rilites. The ancient Cephalopods seem indeed to 

 have here arrived at their maximum of develop- 

 ment, and to have become gradually extinct. The 

 Gasteropodous forms comprise species belonging 

 chiefly to existing genera, as Rostellaria, Trochus, 

 Turbo, Vermetus, Auricula, and Dentalium, and 

 the fossil genus Cirrus. The bivalves consist prin- 

 cipally of Plagiostoma, a fossil genus allied to Spon- 

 dylus, with the valves beset with spines, and which 

 is very frequent in the Chalk ; the thin fibrous Ino- 

 ceramus ; the spiny Podopsis ; and the extinct ge- 

 nus Gatellus are also found j besides Ostrea, Corbis, 

 Thetis, Perna, Gryphcea, Trigonia, and Gervillia. 

 Pectens are also common, as is also Rhynchonella, 

 one of the same family. 



.The Chalk-formation is most numerous in the 

 forms of the free Echinoderms, as the fixed, pedun- 

 culated genera were more abundant in the older 

 secondary rocks. Spatangidce prevail in the lower 

 Chalk ; we find Holaster, and smooth Spatangi, and 

 species of Micraster and Ananchytes, or Helmet 



