EXPLANATION OF THE PLATES AND WOODCUTS. 



This figure is taken from the specimen presented by the Rev. H. Sykes to the Museum 

 of the Philosophical Institution of York, where it is now preserved. The opportunity of 

 describing and figuring this interesting specimen was afforded me, subsequently to the 

 communication of the papers on the Thylacotherium Prevostii and Phascolotherium Buck- 

 landi (pp. 45 and 57), by my friend Prof. PhiUips, to whom I beg to return my best 

 acknowledgments. This specimen, which, like the others, is from the oolitic slate of 

 Stonesfield, is more perfect in regard to the teeth than either of the above-described jaws 

 of the smaller species of Thylacotherium. That it belongs to this genus is proved by the 

 number of teeth, and there is a close correspondence in the form of the jaw; but the dif- 

 ference of size is greater than has been observed in mature individuals of the same species 

 of Placental or Marsupial Insectivores. I therefore indicate the species which this fossil 

 represents under the name of Thylacotherium, Broderipii, in honour of the geologist and 

 naturalist to whom we are indebted for the first description and figure of the Stonesfield 

 mamniiferous remains. 



In this, as in the two preceding instances, it is the left ramus of the lower jaw which 

 offers its inner surface to the observer : it presents at its anterior part the sockets of three 

 incisors and one canine, of small and nearly equal size, each having a simple fang ; then fol- 

 low the empty s.ockets of three small false molars, each with two fangs ; to these succeed 

 three large false molars, in place, each having two fangs protruded to a certain extent from 

 their sockets, and fixed by the adherent matrix in that position, which proves that they 

 were not anchylosed to the osseous substance : these teeth became loosened and displaced 

 doubtless after decomposition of the soft parts ; and the anterior teeth, which are missing, 

 were probably lost from the same cause, before the jaw was finally encased in the oolite. 

 There is a small anterior as well a? posterior tubercle at the base of the large middle cusp 

 or cone, in each of the three premolars which are in place : the middle cusp of the pos- 

 terior one is fractured : there is a slight ridge along the inner side of its base in that 

 tooth, indicating the transition to the true molar series. The first true molar is wanting ; 

 the next four present the inner surface of their crowns in a perfect and uninjured state : 

 the large middle cusp has a smaller one at the anterior and posterior part of its base ; 

 this is traversed by a strong ridge along the inner side, which supports three small cusps ; 

 one of these rises at the middle of the base of the large external cusp, and the other two 

 form the anterior and posterior extremities of the crown of the tooth. This form of grinder 

 resembles that of the Phascolotherium, except in the presence of the middle internal cusp, 

 more than that of the molars of the true Bidelphys. The condyloid and coronoid pro- 

 cesses have both left their impressions on the matrix : the angle of the jaw is fractured : 

 there is the same shallow, wide and smooth groove near the lower margin of the jaw, and 

 the same notch in the symphysis. 



Fig. 2. Inside view of the right ramus of the lower jaw oi^ Phascolotherium Bucklandi; 

 from the specimen in the British Museum. Didelphys Bucklandi : Broderip, ' Zoological 

 Journal,' vol. iii. p. 408, pi. xi. ; Buckland's ' Bridgewater Treatise,' pi. ii. a. 



The outline figure represents the jaw of the natural size. 



In both the figures a indicates the condyle ; b, the coronoid process ; c, the angular 

 process of the jaw ; d, the inferior longitudinal groove. 



