PURBECK FORMATIONS. 35 



Species 2. — Peralestes (Phascolestes 7) longirostris, Owen. Plate II, figs. 4, 4 a, b, c. 



From the size and shape of the last two premolars and the contrast they present to 

 the small succeeding molars, together with the shape of so much as is preserved of the 

 crowns of these, which show a corresponding similarity with the lower molars of Sarco- 

 philus, with that indicated in the upper molars of fig. 3, I am disposed to refer the 

 specimen to be described (PI. II, fig. 4, nat. size, fig. 4 a, magnified 3 diam.) to the same 

 genus ; although, it is true, that there are indications of a greater number of true molars 

 than in the upper jaw of Peralestes longirostris. 



Since, however, in Myrmecohius, the only known existing form of multidentate 

 Marsupial, there is one more true molar on each side the lower jaw than in the upper 

 jaw, I am unwilling, especially as the actual teeth are not preserved in the mandible 

 under consideration, to separate it from the genus showing so marked a resemblance in 

 the proportions of the contiguous premolars and molars, and so close correspondence 

 in the characters of upper and lower molars with those in Sarcophilus. If an upper jaw 

 should eventually be found showing seven or eight true molars of Peralestian type, fol- 

 lowing the large cuspidate premolar, the generic name Phascolestes might be accepted 

 for the species represented by the present mandible. 



The specimen is the anterior half of a left mandibular ramus, showing the symphysis 

 and inner surface with an impression of the posterior part as far as the beginning of the 

 coronoid process. 



The teeth in place are the four incisors («', i, 2, 3, 4), the canine (c), four premolars 

 {p 1, 2, 3, 4), the basal part of the erown of five succeeding teeth {m i, 2, 3, 4, 5), and 

 the impressions of three successive long and slender cones, which, if belonging each to a 

 distinct tooth, would make the premolar-molar series twelve in number on each side of 

 the lower jaw, which is that in Amphitherium (PI. I, fig. 23). 



The present specimen is fully one fifth longer and is proportionably deeper at the 

 corresponding part than the jaw of the type oi AmphitJierium there figured (and in the work 

 cited below^) ; it has a longer and larger canine, and also unequivocally shows four incisors 

 in situ, making eight in front of the lower jaw. These incisors, instead of being arranged 

 transversely at right angles with the canine and molar series, as in Thylacinus and 

 Lang ar us, form a series of sockets curving gradually forward to that of the foremost, due 

 to the symphysis, as in Amblotheriwn, Amphitherium, Phascolotherium, and Myrmecohius. 

 As in the latter Marsupial, also, the foremost incisor is the largest, and is subprocumbent in 

 position. The last three incline rather forward, but less so than does the first. The second 

 incisor is the smallest ; the third and fourth increase without gaining the dimensions of the 

 first. Each stands apart from the other by a short interval, as in Myrmecohius. They are 



1 'History of British Fossil Mammals and Birds,' 8vo, 1846, fig. 15. 



