crotalidæ

from The Century Dictionary.

• A family of venomous serpents, of the group Solenoglypha of the order Ophidia, having a dilatable mouth with perforated poison-fangs, and poison-glands, and differing from Viperidæ chiefly in having a deep pit on each side of the head between the eye and the nostril, whence they are also called Bothrophera; the rattlesnake family: so called from the crepitaculum or rattle with which the tail ends in many of the species.