12 NEW-YORK FAUNA — MOLLUSCA. 



SECTION 2. INFRABRANCHIA. 



With nearly the same form and organization as in the preceding section ; but their gills, 

 instead of being placed on the back, resemble one or two long series of laminae under the 

 mantle, either surrounding the body, or on the right side only. One or two pair of tentacula. 

 Occasionally an external or internal shell. 



Obs. This section has been subdivided into two families, viz. Phyllidia, where the branchiae 

 are on both sides, and no shell is present ; and Hemi phyllidia, where the gills are on the 

 right or left side only : sometimes with a shell. To this latter we refer the following genus. 



GENUS ANCYLUS. Midler. 



Animal oval. Head large, with two large cylindrical contractile tenlacula ; the eyes placed 

 at their internal bases, and with a contiguous foliaceous appendix on the outer side. Mouth 

 beneath. Foot large, elliptical. Gills in a cavity on the left side, between the mantle and 

 foot. Shell patelliform, obliquely conical. Apex inclining forward and to one side. Aper- 

 ture more or less oval. 



Obs. The true position of this genus is yet far from being well established. It cannot, 

 however, well be arranged with the other freshwater mollusca, inasmuch as it is branchi- 

 ferous, whilst they are pulmonous. The animals of this genus abounds in freshwater streams 

 and ponds, climbing over stones and aquatic plants. We enumerate the following species. 



Ancylus rivularis. 



PLATE V. FIG. 98. a. b. — (STATE COLLECTION.) 



Ancylus rivularis. Say, Nich. Encyc. ; Journ. Acad. Nat. Sc. Vol. 1. p. 124. 

 A. id. Gould, Invert. Massachusetts, p. 224, fig. 153. 



Description. Shell corneous, opake, small, narrow. Apex obtuse, almost central, nearer 

 to and leaning towards one side and one end. Aperture oval, somewhat narrower at one end. 

 Color. Greenish or dark green, with a dull brown epidermis; within, milk white or brown. 

 Length, 0-2-0-25. Height, • 1 . 

 Common. Adhering to stones and aquatic plants in streams and ponds. 



