INTRODUCTION. XIV 



animal, principally to the shell-muscle, by which the animal can 

 move it in every direction, protrusive or otherwise. These 

 muscles have been termed the anterior, lateral protractors, and 

 pharyngeal or buccal retractors. 



ODONTOPHOEE AND JAW. 



Just within the circle of the mouth is the buccal cavity, and on 

 the upper anterior side is the solid chitinous jaw set in the strong 

 mandibular muscles. In the genera treated of in this work there 

 are only a few in which the jaw is absent ; in those possessing it, 

 it is sometimes solid, sometimes thin and of oxygnathous type, 

 i. e. with a sharp cutting-edge. The jaw serves to press down 

 upon, hold, or even cut off what the animal may feed upon. 

 Within the buccal cavity on the lower side is a cartilaginous 

 globose cushion on the floor of the mouth, the odontophore ; its 

 upper surface is covered with a tough, long, ribbon-like membrane, 

 set with rows of minute, recurved, chitinous teeth, extending 

 backward to the radular sac. In this last the teeth are formed 

 and pass gradually forward as the rows in front become worn 

 away. These teeth help, with the edge of the jaw above and 

 the action of the lingual protractors and retractors, to the 

 rasping off and breaking up of food. The surface of the radula 

 can be folded together, as in the state of rest, or spread out 

 laterally over a plane surface. The odontophore is developed 

 early in life in some ovoviviparous forms ; the radula is found 

 complete in the embryo enclosed in the eggs lying in the 

 oviduct. 



The teeth of the radula are arranged in rows, generally about a 

 hundred the rows either nearly straight or forming with the 

 central tooth at the apex an obtuse angle. The central tooth 

 and its neighbouring admedians are usually set upon thin, broad, 

 quadrate plates, which merge gradually into lateral teeth rising 

 from much narrower plates. The cutting-tooth is sharp-pointed 

 and raised well above the basal plate, the whole series forming a 

 most perfect natural file. 



In carnivorous genera the teeth are aculeate or scimitar-shaped, 

 rising from elongately ovate bases. 



Perhaps one of the most interesting morphological characters 

 to be noticed in the radula of these Eastern slug-like mollusks, 

 comprising even some with Helicarion-like shells, is the existence 

 of two very distinct groups or subfamilies, one with a Macrochlamys- 

 like radula, the other with a very different polydont one of several 

 hundred teeth, all similar in form throughout on very narrow 

 basal plates, and rarely having any large adrnedian, though a few 

 intermediate forms occur. 



ORGAN OF EESPIRATION. 



The respiratory orifice is distinctly seen on the right side of 

 the body, lying in a dextral shell near the upper inner angle of 

 the aperture, in a slug-like mollusk it is on the right side, in both 



