The Collaborative International Dictionary of English v.0.48:

Turtlehead \Tur"tle*head`\ (t[^u]r"t'l-h[e^]d`), n. (Bot.)
   An American perennial herb (Chelone glabra) having white
   flowers shaped like the head of a turtle. Called also
   snakehead, shell flower, and balmony.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Chelone \Che*lo"ne\, n. [Gr. chelw`nh a tortoise. So named from
   shape of the upper lip of the corolla.] (Bot.)
   A genus of hardy perennial flowering plants, of the order
   Scrophulariace[ae], natives of North America; -- called
   also snakehead, turtlehead, shellflower, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

turtlehead
    n 1: showy perennial of marshlands of eastern and central North
         America having waxy lanceolate leaves and flower with lower
         part creamy white and upper parts pale pink to deep purple
         [syn: shellflower, shell-flower, turtlehead,
         snakehead, snake-head, Chelone glabra]