OP CONCIIOLOGY. 15 



with the opening as previously described to the right of the head, 

 in the mantle margin. The buccal organs and viscera are sepa- 

 rated from the chamber by a tough membrane, which lines the 

 latter. The lung is leaf-shaped, and included between the lining 

 of the chamber and the inner surface of the mantle ; to the left 

 of the lung between the membranes are a series of solid, flat- 

 tened lobes, which empty by a duct near the pulmonary papilla. 

 These are analogous with the renal organ of ^lelampus. They 

 do not project into the chamber, nor have they any resemblance 

 to a gill. The lung terminates in a bifid papilla just within the 

 opening in the mantle. In the posterior part of the lung is a 

 capsule which contains the heart. The suture between the au- 

 ricle and ventricle is very strongly impressed. The heart is 

 closely connected with a network of large blood vessels, from 

 which branches penetrate the tissue of the lung, and the smaller 

 capillaries anastomose between the membranes beyond the edge 

 of the lung in the most delicate and beautiful manner. Dr. 

 Leidy, who examined the organ, had no doubt as to its being a 

 true lung. 



Several large vessels supply the liver and buccal region, and 

 the smaller vessels enter the foot below the buccal mass. 



Alimentary System, The mouth is transversely oval, capable 

 of considerable dilatation, with numerous rugae, parallel with 

 the axis of the throat, when contracted. There is no jaw. The 

 buccal mass is smaller and less muscular than in most pulmon- 

 ates. 



The radula contains about four hundred rows of teeth ; the 

 lateral rows forming a sharp angle with the rhachis. The for- 

 mula is about llO-1-llO, so that the'odontophore contains about 

 88,400 teeth. The nearest relations of the dentition appear to 

 be with Siplionaria. The rhachidian tooth is minute, incon- 

 spicuous, with an ill-defined cusp. The outline of the base is 

 hardly perceptible. The cusp is short, oval and pointed. The 

 edges of the base are slightly thickened. The bases of the inner 

 laterals are obliquely rhombiform. The cusp consists of one 

 long prominent tooth, flanked on each side by a slender, needle- 

 like, minute, transparent denticle. These are very liable to be 

 overlooked from their extreme transparency and small size. 

 They are shorter, proportionally, toward the edge of the radula. 

 Occasionally the main point of the cusp is bifid, or the smaller 

 points are blended more or less with it; these forms, however, 

 are evidently abnormal. 



(It is noteworthy in cases, such as this, when the radula con- 

 tains a large number of similar and minute teeth, that the ten- 

 dency to variation among them is much greater than when the 



