AND ALLIED FORMS. 13 



vibrated laterally until their tips met in front, and tlicy inclosed 

 a heart-shaped open space between their margins. Under some 

 circumstances they are doubtless used as tactile organs. The 

 anterior extremity of the foot, between the tips of the auricles, 

 is bi-marginatc, or divided into two lips by a slit or furrow of 

 slight depth. The subtruncate posterior extremity of the foot 

 sometimes takes an obscurely trilobate form, the middle lobe being 

 broad and arcuated, while the small lateral lobes are dentiform. 

 The operculigerous lobe is oval or rounded, not continued ante- 

 riorly, but broader than the foot, so that it projects on either 

 side beyond the margin of the latter. The operculum is thin, 

 horny, and subspiral, with about two turns and a half. It is 

 striated, both longitudinally and transversely to the whorl, except 

 at the outer or larger extremity, where the transverse lines only 

 are apparent, and are different in direction from the others. There 

 are two parallel areas at the outer or dorsal margin, the inner 

 one being longitudinally and the outer one obliquely 

 striated. See Fig. 3. It is proper to state that this Fig- 3. 

 figure is taken from the Massachusetts species, A. 

 limosa according to Haldeman. 



The rostrum is very short, but normally placed in 

 advance of the foot in consequence of the anterior 

 position of the head ; it is broader than long, and 

 emarginated at the middle of its antero-inferior edge. The ten- 

 tacles are slender, very long, two-thirds as long as the foot, exactly 

 cylindrical, and blunt or truncated at their tips. The eyes are 

 placed just at the outer bases of the tentacles, on the anterior 

 side of somewhat pnominent tubercles or bulg- 

 ings of this part of the head. The mantle Fig. 4. 



edge is simple. The gill, a portion of which 

 is represented in Fig. 4, is situated in the usual 

 position on the inner surface of the mantle, and 

 is rather broad, far broader than in the Yivipa- 

 ridse and Melauiida?, and consists of transverse laminae of a some- 

 what triangular form with the prominent apices bent over to the 

 left* 



' Mr. Haldeman, in his " Monograph of Amnicola," page 6, attributes to 

 the genus " about 8 rows of pectinated branchiae." I am unable to compre- 

 hend what is referred to here ; perhaps the branchial lamins themselves 



2 



