Hydrobia totteni (=Hydrobia minuta) is considered a distinct species and not synonymous with the European species Hydrobia ventrosa. Of the European species of Hydrobia, H. totteni is most closely related to H. ventrosa on the basis of penial structure, dorsal head pigmentation and radula formula. The species is considered distinct in that it has fewer rows of teeth on the shorter and narrower radula. H. totteni has a hypertrophy of the cilia on the left tentacle not shown for H. ventrosa. The tentacles of the former have distinct pigment spots which are not well defined in the latter. There is an apparent difference in structure of the basal process of the central tooth. Hydrobia totteni has fewer cusps on the marginal teeth than H. ventrosa. Other structures not studied or compared but of value for specific observation are the female reproductive system and nervous system. The paper deals at length with the morphological description of the common American East Coast species. Noted throughout are features distinguishing this species as a representative of the subfamily Hydrobiinae from genera of the subfamily Pomatiopsinae as the latter was defined by STIMPSON (1865) and DAVIS (1965).