PURBECK FORMATIONS. 29 



The inner side of the ramus is marked by the Hnear groove continued further forward 

 than is indicated in the specimen (PI. I, fig. 38, (f) of Spakicoihenum tricuspidens . The 

 vertical extent of the horizontal ramus is preserved as far forward as the socket of the 

 canine, both upper and lower borders being nearly straight and parallel. 



§ VII. Genus — Amblotherium,^ Owen. 



In the type of the present genus (PI. II, fig. 1, nat. size and 1 a, magn. 3 diam.) the 

 true molars do not exceed in number those of the lower jaw in Amphitherium (PI. I, 

 fig. 25), and of the upper jaw in Peralestes (PI. II, fig. 3) ; but the incisors are four in 

 each mandibular ramus, equalling in number those in Phascolestes (PI. II, fig. 4). The 

 two hind premolars have crowns exceeding in size those of the succeeding molars. 



The present species, with the full or adult complement of teeth, is markedly smaller 

 than any of the foregoing Mesozoic mandibles save Spalacotherium minus. 



Species 1 — Amblotheritjm soricinum, Otoen. Plate II, figs. 1, 1a, Ib. 



This species is founded on a right mandibular ramus, the inner side exposed (PI. 

 II, fig. 1, nat. size ; 1 a, magn. 3 diam.) showing four incisors (i i — 4), the canine 

 (e), four premolars [p 1—4), and six molars {m i —6), with a trace of what may have been the 

 alveolus of a seventh molar. 



The condyle {b) projects midway between the summit of the coronoid (c) and the angle 

 of the jaw {a), its lower end being on the level of the alveolar outlets. A thin ridge {a), or 

 rather the base of one showing fracture, projects inward from the lower border of the 

 ascending branch, bounding below the groove leading to the entry of the dental canal {d). 

 From the fore part of this entry the groove (y) begins, which extends as a hnear fissiu-e, 

 simulating a suture, from a little above the rounded lower border of the ramus to the hind 

 part of the symphysis (r, s).- 



The hind border of the coronoid process descends from the fine recurved point (c) in a 

 bold or deep regular concave ciu've to the condyle, as in Amphitherium and Fhascolotherium, 

 but its apex does not reach so far back. The state of preservation of the articular process 

 shows clearly enough its convexity, but not its precise size ; it is somewhat crushed, and 

 appears larger in the specimen than it was in the recent bone. 



' 'a/ij3\oiij, to abort ; dijpiov, beast. 



2 It is remarkable tbat a groove corresponding in position with that lodging the mylohyoid nerve and 

 vessels in the human mandible, but of which no trace exists in most lower placentals and existing marsu- 

 pials, should reappear, as it were, so distinctly, in these small Mesozoic mammals. It may be questioned 

 whether it was due to the same mechanical cause and relations as in man ; whether it may not rather 

 indicate a more definite " spleuial" harmonia, remaining longer open than in later mammals ; but which is, 

 nevertheless, entirely closed in all the subjects of the present Monograph. 



