OOLITIC FORMATIONS. 13 



and the remaining six to the true molars. The crowns of the fourth, fifth, and sixth 

 premolars are entire, and consist of a single compressed conical cusp, with a minute 

 tubercle at the hind part of its base, and a more minute one in front ; the base of the 

 crown is slightly tumid, and from it are continued, without the intervention of a cervix, the 

 two slender and shghtly diverging roots. The fractured crown of the first true molar 

 shows more distinct anterior and posterior basal cusps ; those of the second and third show 

 an increased thickness. The fourth gives a view of the anterior cusp, of the large middle 

 cone, and of part of the posterior cusp ; the thicker and more complex crowns, as com- 

 pared with those of the premolars, are unequivocally shown in the last three molars. 

 The roots of the teeth are seen in the specimen, fig. 21, to descend half way or more toward 

 the lower border of the ramus ; their substance is contrasted by its denser texture and 

 deeper colour with the surrounding bone, from which the tooth-roots are separated by a 

 thin layer of a distinct substance, infiltrated apparently from the matrix into the sockets. 

 In most Reptiles the base of the fully developed tooth is confluent with the bony 

 substance of the jaw ; in the few in which the implanted base remains distinct it is 

 simple ; in both cases, with a series of seven contiguous teeth exposed as in the jaw 

 in question, the germ of a successional tooth would be found beneath some of the teeth. 



The broad, elevated, slightly recurved coronoid process resembles that in Didelphys, 

 Basyurus, Perameles, Erinaceus, and the like small predatory mammals ; the position of the 

 condyle, on a level with the teeth, is also a character of a feeder on animal substances. 



The position and form of the entry of the canal {d) transmitting the nerve and vessel of 

 the teeth accord with the mammalian type of the jaw. The (mylohyoid) groove is present 

 in the jaw of Myrmecohius (fig. 24, g) ; its depth and length are greater in the fossil. 

 Comparative anatomy supports the inference that the Stonesfield fossil examined by Cuvier 

 belonged to a small ferine' mammal with a jaw much resembling that of an Opossum, but 

 differing in the great number of the molar teeth, in this respect exceeding the Myrme- 

 cohius, PI. I, fig. 24), in which they are nine in number. 



Four names have been proposed for the Mammalian genus represented by this jaw ; 

 of these Am.phitherium is the one adopted in my ' History of British Fossil Mammals/ and 

 which I here retain. 



The second fossil of this species (PL I, figs. 22, 22 a) — also a ramus or half of the 

 mandible — discovered in the Stonesfield slate, supplied additional evidence of the osseous 

 structure. It is preserved in the Geological Museum at Oxford, and is described and 

 figured in my Memoir of 1838.^ With the exception of parts of the coronoid, condyloid 

 and articular processes, the exposed inner surface of the ramus is entire; the symphysis {s) 



1 I use the word 'ferine' as equivalent to the French 'carnassier,' the term by which Cuvier signified 

 collectively the Cheiroptera, Insectivora, Carnivora, and Marsupialia. 



2 'Geol. Trans.,' ser. 2, vol. vi (1839), p. 49, pi. v, fig. 1. 



