54 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



partition, containing the digeBtive and generative apparatus and the 

 nervous centres. 



Ariolimax and Prophysaon have the same general an*angement as 

 Limax and Avion. Hemphillia is distinguished by having its shelly 

 plate external, its edge lightly imbedded in the mantle. 



In Tehennophoriis and Palliftra the mantle covers the whole upper 

 surface of the body, and encloses no testaceous rudiment. Its anterior 

 edge is unattached, and the head is retractile beneath it. The pulmo- 

 nary chamber is placed beneath the anterior part of it ; and the muscu- 

 lar membrane, bounding the visceral cavity in a great part of its extent, 

 is but loosely attached to the outer integument. 



In Veronicella the body appears broad from the mantle, which en- 

 closes the whole body except the comparatively narrow pedal disk, form- 

 ing a lateral, angular projection as it is inflected inferiorly to the margin 

 of the pedal disk. In transverse section it is semi-elliptical. The man- 

 tle contains no testaceous rudiment. The head can be but slightly 

 protruded. The tentacles are bifid. The respiratory orifice is situated 

 on the right side of the tail, between it and the extremity of the pedal 

 disk. The anal aperture opens at the posterior margin of the latter 

 orifice. The generative apparatus has two distinct external apertures, 

 distant from each other. The male genital orifice is placed just beneath 

 the mouth, between it and the blind sac, inclining to the right. The 

 female orifice is situated upon the inferior part of the left side of the 

 mantle, midway between the head and tail. As usual, the body has 

 two cavities, of which the pulmonary occupies a position at the right 

 posterior part, beneath the mantle, and extending backwards on the right 

 to the tail. 



Onchidium has a similar arrangement to Veronicella, but has no ten- 

 tacles. 



General ReMxVrks on the Terrestrial Testaceous Gasteropoda. 



A testaceous gasteropod resembles a slug with the greater portion of 

 the viscera squeezed out upon the back, and arranged in a turbinate 

 manner. The turbinate mass is always an exact mould of the testa- 

 ceous covering of the animal ; its length in the spiral direction holds no 

 proportion with that of the foot, or that part of the body which the 

 animal protrudes from the shell, and differs very much, not only in dif- 

 ferent genera, but also in different species of the same genus. With an 

 increase in length a proportionate decrease in breadth is observable, 



