MULTITUBERCULATA. 249 



Somersetshire. A detached incisor or canine of corresponding 

 size has also been found in the latter locality. 



The small multituberculate grinding teeth from the Stones- 

 field Slate (Bathonian) of Oxfordshire, named Stereognathus 

 ooliticus, are perhaps placed among Mammalia with a little 

 more certainty ; but only one fragment of jaw is known con- 

 taining three teeth. The dental crowns are provided with 

 crescent-shaped tubercles arranged in three longitudinal series. 



There is still less doubt about the mammalian nature of 

 some upper and lower jaws of Upper Jurassic age, discovered 

 both in Britain and North America, and characterized by multi- 

 tuberculate molars. These form the family Plagiaulacidae, 

 and the best-known specimens are described under the generic 

 names of Plagiaulax, Bolodon, Allodon (fig. 147 G), and 

 Ctenacodon. Except two detached teeth from the Wealden, all 

 the known British specimens have been obtained from tK 

 Middle Purbeck of Swanage, Dorsetshire, nearly all from a bed 

 of marl only a few inches in thickness. 



Fio. 146. 



Plagiaulax minor; right mandibular rairus wanting hinder end, outer aspect, 

 four times nat. size. U. Jurassic (M. Purbeck Beds) ; Swauage, Dorset- 

 shire, m, molars ; p, premolars ; the large tooth in front an incisor. 

 (After Falconer.) 



Plagiaulax (fig. 146). A genus known only by mandibular rami which 

 meet in a loose suture at the symphysis. The mandible is short, with a 

 high coronoid process and a low articular condyle not above the level of 

 the dentition. The condyle is convex and much deeper than broad. The 

 postero-inferior angle of the jaw is inflected. There is a single pair of 

 enlarged closely apposed incisors at the symphysis, apparently not per- 

 sistently growing, and without any specially thickened band of enamel on 

 the anterior face. Then follows a diastema without any trace of a canine. 

 Next are three or four closely arranged cutting teeth, gradually increasing 

 in size to the fourth, which is very large, and all marked with oblique 



