2 EOCENE MOLLUSCA. 



and they are endowed with the sense of smell. The eyes are placed on the sides of 

 the head, and in one of the two orders into which the animals are divided (the Dibran- 

 chiata) are, generally, lodged in an orbital cavity in which they move freely ; in some 

 genera, however, they are united to the outer integument, and are then incapable of 

 motion. When lodged in orbital cavities, they exhibit two distinct modifications, of 

 which M. d'Orbigny has availed himself for the subdivision of the order into two 

 groups (called respectively Myopsidce* and OigopsidmX). In the first modification, 

 which is found among the littoral Cephalopods, the eyes are wholly covered by the 

 skin of the head, which becomes thin where it passes over the ball of the eye ; in the 

 other modification, which characterises the pelagic species, the orbital cavity is largely 

 open, and the eyes are in contact with the water. 



In the tetrabranchiate Cephalopod, whose food is found principally at the bottom 

 of the sea, and to whom enlarged vision would be comparatively useless, the eye is 

 not lodged in an orbital cavity, but pedicillated, and assumes a simpler structure, 

 approaching that of the inferior mollusc. 



These animals possess an external auditory opening, generally protected by an 

 external ear more or less complicated in structure. The organs of smell are supposed 

 to reside in certain pores or sacs, opening externally, termed by M. d'Orbigny 

 aquiferous pores (ouvertures aquiferes), which are divided into cephalic, oral, anal, and 

 brachial pores according to their position, and are used by him as generic distinctions. 

 The mouth is terminal, and is furnished with two strong, horny beaks or jaws termed 

 mandibles, working vertically upon each other like the bill of a parrot, with which they 

 are usually compared. 



The body is inclosed in a thick membranous skin or mantle, united along the belly 

 of the animal so as to form a muscular bag or sac, open at the upper extremity, and 

 containing the branchial apparatus and viscera. In its general shape it is round, or 

 more or less elongated, and cylindrical or depressed. To this body, distinguished as 

 the posterior portion of the animal, the anterior or cephalic portion, consisting of the 

 head and the arms or tentacula it sustains, is attached by one or more ligaments, 

 some internal, others formed by the continuation of the skin of the body, and termed 

 cervical or lateral ligaments, according to their position. The condition of these 

 ligaments varies with the habits of the animal, and upon it generic characters are 

 founded. They attain their greatest strength in the littoral Cephalopods, and in some 

 genera afford a sufficient attachment between the head and the body ; but in the 

 Cephalopods more or less pelagic, in which the free and independent use of the organs 

 of prehension, and a simultaneous active exercise of the bodily locomotive function, 

 would be materially impeded by an extensive permanent attachment, the ligaments 

 are considerably reduced ; so much so, in fact, that they would afford a very imperfect 



* Myopsidac ; from fjivtu claudo, oipis visus. 

 f Oigopsidse ; from oiyu) apcrio, o\pts visus. 



