228 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



and on the palatines and vomers, rarely on the parasphenoid, while 

 they are entirely lacking in Bujo and Pipa. 



Among the reptiles the turtles and some of the pterodactyls are 

 toothless; most of the others have the teeth confined to the margin of 

 the jaws, though they occur on the palatines and pterygoids in the 

 snakes and lizards, and rarely (Sphenodon) on the vomer. While the 

 conical shape prevails, the teeth present a great variety of forms, 

 some of the theriomorphs closely simulating the mammals in their 

 heterodont dentition. The teeth may be anchylosed to the summit 

 of the jaws (acrodont) ; applied to their inner side (pleurodont, fig. 

 io6, d) ; or have their roots implanted in grooves or sockets or alveoli 

 (thecodont). Mention must also be made of the poison fangs of cer- 

 tain 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 



Fig. 240. — Poison gland and fang of rattlesnake, Crotalus horridus (Princeton, 1404). 

 P, poison gland; /, labial glands. 



along the roof of the mouth, but when it is opened, they are brought 

 into position for striking the prey (vipers, rattlesnakes — soleno- 

 glypha) . Correlated with the fijced or movable condition is a modi- 

 fication in the teeth themselves. In the proteroglypha a groove runs 

 along the anterior side of the fang by 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. 240). 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 great variation is found in the teeth of mammals, the hetero- 

 dont 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 



