SPECIAL ANATOMY. 53 



much more, if not absolutely, restricted to one single type of dentition. 

 I do not venture any further deductions at this time. 



I will add that all the figures of dentition in the plates have been 

 drawn by my own hand from the microscope itself, with the aid of the 

 camera lucida. 



IV. SPECIAL ANATOMY 



The following pages are reproduced from the treatise on the subject 

 by Dr. Leidy prepared for Volume I. I have added notes on the more 

 recently discovered genera. 



General Kemarks upon the Exterior Form and Structure of the 

 Terrestrial Naked Gasteropoda. 



Upon examining a Limax or an Avion, we find it composed of a thick, 

 vermiform body, with a broad, ribbon-like, pedal disk, running the 

 whole length of its inferior surface. The anterior obtuse extremity 

 forms the head ; and from it protrude two retractile tentacula, and two 

 retractile eye-peduncles, upon the outer side of the tip of the two latter 

 of which is placed the eye. The mouth is situated at the anteroinfe- 

 rior part of the head ; and immediately below it is a deep depression 

 or blind sac. The posterior part of the body forms the tail, and is 

 acute. Upon the antero-superior part of the body is placed the mantle, 

 which covers the pulmonary chamber, and contains within it a rudi- 

 mentary, laminar, calcareous testa or a congregation of calcareous 

 grains. In other genera these are wanting. The anterior part of the 

 mantle is free and movable, and the head, indirectly through the 

 retractor muscle of the buccal body, is capable of being retracted be- 

 neath it. On the right edge of the mantle the pulmonary orifice 

 exists ; and at the posterior side of the latter the anal aperture is 

 placed. Upon the right side of the head, a short distance posterior to 

 the eye-peduncles of that side, the genital orifice is situated. The body 

 has two distinct cavities, — the pulmonary chamber, containing a vas- 

 cular network upon its surface, the heart, the renal organ, and the rec- 

 tum ; and the visceral cavity, separated from the former by a muscular 



