38 FOSSIL REPTILIA OF THE 



their fangs in the sockets are shown in fig. 7 ; the anterior teeth are narrower 

 than the rest, as in the upper jaw. The crushed or broken state of the specimen 

 at the opposite end prevents a determination of the total number of sockets in this 

 ramus. On the inner side of the specimen (fig. 8), a considerable extent of the 

 symphysis {s, s) is shown. 



The posterior part of a broken and distorted dentary element of the left 

 ramus of the mandible is represented in fig. 9, showing the last eight teeth, 

 and the impressions of the crowns of as many in advance. A portion of the 

 crown, displaced, of the fourth from the last is preserved, and likewise portions 

 also of those in advance, which have been broken in splitting the slab, so that 

 they appear smaller than they actually were. The last three teeth are entire, and 

 show a gradual decrease of size, as in the portion of upper jaw (fig. 4). A mag- 

 nified view of the inner surface of the last lower tooth is given at a, fig- 9. 



From the characters of jaws and teeth above described, the extinct animal pre- 

 senting them might be referred to the modern Lacertian group : but the structure 

 of the vertebrae and limb-bones must be ascertained before the ordinal affinities 

 of Echinodon can be satisfactorily determined. 



The modifications of the mode of implantation of the teeth in the known limits 

 of the Dinosaurian order affect the value of the thecodont character as a mark of 

 affinity. The dentition of Echinodon, in respect to the shape of the crowns of the 

 teeth, appertains to the category embracing Macellodon, Cardiodon* Hijlaosaurus, 

 and Igucmodon. From Macellodon the present genus differs in the swollen borders 

 of the basal half and the stronger serration of the apical half of the dental crown. 

 The similarly expanded crown of the tooth of Cardiodon has thicker and apparently 

 not serrate margins, it is not divided into a basal and apical portion, and the apex 

 is more obtuse. In Hyldeosaurus the crown of the tooth is thicker and less expanded 

 than in Echinodon ; the borders of the apical half are usually abraded by mas- 

 ticatory acts, show no marks of serration, and meet at an angle of 80° ; but the 

 crowns of the teeth were in contact, as in Echinodon. The more complex struc- 

 ture of the teeth of Igucmodon appears, nevertheless, to be due to additions super- 

 posed upon a type of tooth which is essentially like that of Echinodon. The 

 expanded crown is divided into a basal and apical portion ; the marginal serrations 

 of the latter are coextended with the increased thickness of the part into small 

 lamellse, themselves more minutely dentate. The middle longitudinal rising of 

 the enamel, which in Echinodon has appeared to me to be stronger on the outer 

 side of the upper teeth and on the inner side of the lower teeth, is exclusively 

 developed, as the " primary ridge " on the corresponding aspects of the teeth of 

 the upper and lower jaws in Iguanodon. In the small teeth, or those of the 



* From the Oolitic Formation, called " Forest-Marble," near Bradford, Wilts. See ray 'Odonto- 

 graphy,' p. 291, pi. 7^a, fig. 7. 



