386 HYDATINA. 



spicuous color pattern. The animal differs externally in the ten- 

 tacular lobes of the head disk, which are like those of Aplysia, and 

 totally unlike the simple head disk of A keridce. 



Synopsis of Genera and Subgenera. 

 * Animal with four tentacles ; shell without spiral sculpture. 



Genus HYDATINA Schum. 



Shell globose or oval, smooth, with narrow level or concave 

 spire, the aperture broadly rounded below. 



Subgenus APLUSTRUM Schum. 



Shell obovate, with wide spire, the aperture deeply notched at 

 base of the columella. 



** Animal with two tentacles; shell more or less spirally striated. 



Genus MICROMELO Pilsbry. 



Shell globose-oval, with narrow spire, the aperture broadly 

 rounded below ; surface striate-punctate, decorated with spiral and 

 wavy longitudinal lines. 



Genus HYDATINA Schumacher, 1817. 



Hydatina SCHUM., Essai d'un Nouv. Syst. pp. 57, 186, type H. 

 filosa Schum.=physis L. Bullina (in part) Ferussac (see this vol. 

 p. 175). 



Shell globose or oval, thin, smooth, variegated, spirally banded ; 

 the spire exposed, either convex, flat or concave, apex a minute up- 

 tilted globose nucleus. Aperture about as long as the shell ; outer 

 lip not sinused behind, rather effuse or notched at base; columella 

 straight or concave with reflexed edge. Type H. physis. 



Animal capable of complete retraction, large ; head disk bearing 

 four tentacles like those of Aplysia; produced behind in two large 

 lobes partly covering the shell, the eyes at their bases. Foot very 

 broad, auriculate at the anterior angles, obtuse behind ; epipodial 

 lobes lacking ; edges of the large foot reflexed over the shell (pi. 

 45, fig. 17, H. physis). 



Radula with many longitudinal rows of similar teeth ; centrals 

 lacking ; side teeth with denticulate cusps. 



