282 Mr. W. Clark on Ancylus oblongus and A. fluviatilis. 



the oesophagus to the stomach; the oesophagus is inserted a 

 little on one side of the cardia, and when withdrawn by force 

 presents a bluish- white, bulbous, minutely perforated termination. 

 The configuration of the stomach is nearly the same in both 

 species ; it is ridged externally at the cardia, and at one-third 

 of its length from thence becomes slightly constricted, and then 

 expands into a subglobose or bursiform sphere oblate at the 

 pyloric axis ; in the interior it is furnished with muscular raised 

 fillets, and is usually filled with a white pulp ; the duodenum 

 and intestine make about a fold and a half in the liver, ter- 

 minating as rectum close to the respiratory cavity ; these organs 

 are of a pale brown colour, finely circularly striated, and when 

 placed in extension are less than half an inch long ; they have no 

 twists, cseca, or inflations ; the stomach is of a brownish-gray ; the 

 liver is dark brown, and more granular than in the last species. 

 The tentacula and eyes scarcely differ : in this, the mantle is 

 plain, and without the comma-shaped points of the A. oblongus. 

 The verge is white, long, large, fleshy, subcylindrical, retractile, 

 issuing under the left tentaculum ; the orifice of the matrix is 

 quite as distant from it as in its congener, being close to the 

 respiratory vault, and accompanied by a minute vesica. The 

 mantellar collar is a gently inflated cord, expanding on the 

 right and left hand, according as the condition is dextrorsal or 

 sinistrorsal, into a small, oval, flat fillet, which the animal ele- 

 vates when it respires or evolves the rejectamenta. In both, the 

 rectum and canal of the viscous lobe debouche in close connexion 

 with the respiratory cavity, which is situated about one-third 

 from the posterior end, so that the interval of separation be- 

 tween the matrix and verge is nearly at opposite extremities. 

 The pulmoniferous cavity displays, though only with high 

 powers, the heart, auricle, and principal veins, as in its fellow. 

 The lobular salivary glands, and the brain, of two principal and 

 smaller ganglia, differ but slightly ; here the nervous masses are 

 more concentrated. All these points, with those not mentioned, 

 as the ovary, testis, vas deferens, are so essentially alike, as to 

 render distinction only amenable to very insignificant spe- 

 cialties. 



If we compare our Ancyli with one of the principal Limnea- 

 dan types, the L. pereger, we shall find the generalities sub- 

 stantially the same; and in respect of the specialties, the stomach 

 of the ' pereger' is more contracted and taper at the cardia, with 

 the constriction lower, and the intestine more than proportion- 

 ately longer. The mantellar collar is also more firmly pro- 

 nounced, in consequence of the shape of the aperture. As to 

 other external variations, the head may be flatter, the tentacula 

 broader and more membranaceous, and the shell of a greater or 



