DIGESTIVE ORGANS. 213 
(acrodont); applied to their inner side (pleurodont, fig. 97, d); or 
have their roots implanted in grooves or sockets or alveoli (thecodont). 
Mention must also be made of the poison fangs of certain serpents. 
These are specialized teeth borne on the maxillary bones and are either 
permanently erect (proteroglypha) or the bone may turn, as on a pivot, 
so that when the mouth is closed the teeth lie along the roof of the 
mouth, but when it is opened, they are brought into position for striking 
the prey (vipers, rattlesnakes—solenoglypha). Correlated with the 
fixed or movable condition is a modification in the teeth themselves. In 
the proteroglypha a groove runs along the anterior side of the fang by 
Fic. 215.—Poison gland and fang of rattlesnake, Crotalus horridus. (Princeton 1404) 
p, poison gland; J, labial glands. 
which the poison is conducted from the poison gland into the wound. 
In the solenoglypha the groove is rolled into a tube with openings near 
the base and apex of the tooth (fig. 215). In these solenoglyphous 
snakes only a pair of fangs are functional at a time, but there are 
reserve teeth which can come into use on the loss of the first. 
The greatest variation is found in the teeth of mammals, the heter- 
odont dentition being the rule. Four kinds of teeth are recognized. 
These are the incisors in the premaxillary bones, followed by a single 
canine at the anterior end of each maxillary bone. This resembles 
the incisors and differs from the other maxillary teeth in its conical shape 
and single root. Behind the canines come the premolars (the bicus- 
pids of the dentists) which have two roots and complicated crowns and 
appear in both milk and permanent dentitions.. Lastly are the molars, 
like the premolars in form, with several roots, but appearing only in the 
permanent dentition. The corresponding teeth in the lower jaw have 
the same names. 
In a few mammals, like the whales, all of the teeth are of a simple 
conical shape, but in the majority the crown of the molars is marked 
by projections—cones, tubercles, crests, etc.—which are variously 
