X 



SYNOPSIS, 



&c. &c. 



MOLLUSCA. 



The MoUusca, or as anatomists more correctly call tliem, the Heteror/anr/liata, include all the sliell-fish, together 

 with several naked groups of little interest to the geologist. The principal divisions of the class are, 1st. The 

 Cephalopoda, which are the most highly organized, active, and rapacious of the entire group, having a distinct 

 head, furnished with powerful horny mandibles, large, perfectly-formed eyes, a rudimentary skull and brain, 

 powerful fins for swimming, and several other pecuharities of structure only found in the Vertebrata, to which 

 they lead by means of the Fishes; the sexes are distinct. 2nd. The Gasteropoda, including all the spiral 

 univalve shells, together with some naked groups, but all characterized by having the foot expanded into 

 a disc for walking ; they have also a head, and two eyes, and usually two or four tentacula; sexes distinct. 

 3rd. The Dit/u/ra, or bivalves; those are much less highly organized than either the Cephalopoda, or Gastero- 

 poda, having no head, tentacula, or eyes, and scarcely any powers of locomotion, having no fins for swimming, 

 or disc-like foot for walking; many of the genera are permanently attached to foreign bodies, either by a 

 byssus, or by the substance of the shell ; they are all females. 4th. Brachiopoda, or arm-footed bivalves, of 

 which we shall speak more when we come to them. There are, also, the Pteropoda and Tuidcata. 



1st. We have 

 Fig. 1. 



CEPHALOPODA. 



The Cephalopoda possess the power of swimming in greater perfection than any other Molluscous animals, 

 and are purely carnivorous in their habits ; they are provided with large arras or feet, which surround the head, 

 and with which they can walk, seize their prey, and swim. There are several distinct groups, 

 the Dihranchiata (fig. 1) or naked cuttle fishes, with two gills and three hearts ; they possess 

 the power of grasping, in a very remarkable degree, most of the species having two long 

 fleshy arms, furnished with numerous suckers for the purpose ; they arc most highly or- 

 ganized, and approach nearest to the Vertebrata in size, structure, appearance, and habits ; 

 they can swim with tolerable velocity by means of fleshy fins, are very rapacious, and 

 have the senses of hearing, seeing, and smelling, in higher perfection than any others ot 

 the class ; besides the cranial cartilage, or skull, they have a large dorsal internal support, 

 which though really analogous to the internal shell of a Limax, and having no organic 

 connexion with the soft parts of the animal, still forcibly reminds the observer of the dorsal ^ 

 spine of the skeleton of the Vertebrata, to which, in their organs of- nutrition, prehension, y^ 

 respiration, circulation, and the brain and nervous system, and organs of sense, they ap- 

 proach more nearly than any other of the molluscous group. 2nd. We have the Sipho- 

 nifera, or Tetrabranchiata, including all the polythalaraous shells which have their chambers connected by a 

 siphuncle. Those appear to have been of the highest importance in the earlier periods of the earth's history, the 

 immense multitudes oi Ammonites, Baculites, Scaphites, Hamites, Orbidites, and Nautili, which crowd the 



B 



