Description: Extinct mako shark (Isurus hastalis) broad-form upper anterior tooth (lingual view). I. hastalis teeth occur in two forms, broad-form and narrow-form (there are theories, of course). The upper anterior teeth in I. hastalis are more curved than the lowers. The characteristic distinguishing this species from the extant I. retroflexus is the increased width of the basal groove dividing the root from the crown near the center of the tooth. Other Isurus species have a basal groove of nearly constant width.