Eight species of gastropods of the family Turridae from the middle Miocene St. Mary's Formation are redescribed and figured. Large members of specimens were collected from 4 localities in Maryland, and a quantitative study was made to determine their taxonomic significance. The anal sinus characters were found to be the most useful in species characterization. Several previously separated species and varieties are combined into 1 species in both Spendrillia limatula (Conrad) and Clathrodrillia? gracilis (Conrad). There is evidence that some previously used generic and specific characters vary considerably within a single species. The species formerly assigned to Surcula and Drillia belong in various other genera.