251 



of Its own length. Inferiorly, the entrance of this projecting portion 

 of the smaller shell is closed by a flat septum, extending from margin 

 to margin, and continuous anteriorly with the dorsum and internal 

 shelf of the larger shell presently to be described. 



The larger shell is thin, translucent, presenting signs of rapid growth, 

 and usually of a whitish or very pale horn color. It is more expand- 

 ed to the left than to the right, the dorsum and left slope being strong- 

 ly convex, while the right slope is nearly straight. It is marked with 

 prominent stride of growth, and indistinct radiating lines. Within, 

 at the narrower posterior end, there is a rather strong white shelf, 

 formed by the soldering of the dorsum of the larger to the septum of 

 the smaller shell, which extends forward and upward, nearly to the 

 bottom of the concavity, leaving, however, an aperture which leads 

 into the cavity of the smaller shell, in which the liver of the animal is 

 seated. This aperture is exactly semi-lunar in shape, its longer diam- 

 eter being of course coincident with the width of the smaller shell, and 

 ■equalling about one-third that of the larger shell. In younger speci- 

 mens the shelf is a little less extensive, and the apicial aperture some- 

 what larger. 



The soft parts of the animal, except in the form of the visceral sack, 

 agree so closely with those of true A ncyli that I have not succeeded in 

 finding any differences of importance. I add here a figure of its lin- 

 gual dentition. This resem- Fig. 3. 

 bles very nearly that of a spe- 

 cies of Ancylus common in 

 the District (which appears 

 to be the A . rivularis of Say 

 and Haldeman), differing from it only in having two or three teeth less 

 in number, and in the more numerous denticles with which its lateral 

 teeth are armed. 



After a close examination of the above characters, I have ventured 

 to suggest that the Gundlacliia commences its life as an Ancylus ; the 

 smaller shell, in which the earher period of its life is spent, being undis- 

 tinguishable in form fi'om the shells of that genus. It is probable that 

 it passes the first summer and autumn of its existence in this smaller 

 shell and that the septum which afterwards partially closes its aperture 

 is formed during the period of inaction, which ensues during the winter.* 



* It has now reached the state in which G. ancyliformis was originally de- 

 scribed by Pfeiffer, in 1849, and which was then thought to be the adult form. 



In making the above remarks, I am well aware that Bourguignat, in the Revue 

 et Magasin de Zoologie tor 1862, p. 19, asserts that this septum, or " inferior la- 

 mella" exists in the young Gundlackia ancyliformis from the date when it leaves 

 the egg, and savs that he has even been able to verify this as a fact. Without 

 wishing to throw doubt upon so positive a statement, I may be permitted to sug- 



