64 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 20, 



which affects the enamelled summit of the whole ridge in Lophi- 

 odon, abrades only the summits of the inner cones in Pachynolo- 

 phus and Pliolophus ; moreover the oblique ridges in Pachynolo- 

 phus appear from M. Gervais's figure to dilate a little in breadth at 

 their beginning, but this swelling is not so marked and circumscribed 

 as in Pliolophus, and consequently an intermediate island of enamel, 

 as at r and s, PL III., between an outer and inner cone, is not pre- 

 sented in any of the molars of Pachynolophus, although the first of 

 these, m i, in the specimen figured by M. Gervais, has been as 

 much worn down as in the corresponding molar of Pliolophus. In 

 this respect Pliolophus presents the next transitional step in the 

 passage from the type-dentition of Lophiodon to that of Hyraco- 

 therium, in regard to the modification of the working surfaces of 

 the molar teeth. 



The hinder half of the last molar, m 3, presents a minor area, as 

 in Pachynolophus, and a more simple configuration ; the ridge from 

 the postero-internal cone being simple, not expanding into an acces- 

 sory tubercle. 



M. Gervais calls attention to the seeming quadrilobate character 

 of the outer side of the crown in the true molars of Pachynolophus, 

 produced by the development of the cingulum into a tubercle at the 

 fore and back part of that side of the tooth. Pliolophus resembles 

 Pachynolophus in the tubercle at the fore part of the outer wall, 

 but the cingulum is not so expanded at the back part as to give the 

 appearance of a fourth cone. In this respect Pliolophus resembles 

 Lophiodon proper. 



In regard to the lower jaw, the lower contour of the symphysis is 

 in the same line with that of the lower border of the ramus in Pa- 

 chynolophus, and the symphysis with the incisor teeth are more pro- 

 cumbent even than in Pliolophus : the diastema between the pre- 

 molars and canine is twice as long, and the consequent modification 

 of the mandible led M. Gervais to propose the specific name lepto- 

 gnathum for the small Lophiodont of the ' Calcaire grossier,' which 

 M. Pomel had previously dedicated to his friend M. Duval. But a 

 distinction of more decided generic importance between Pachyno- 

 lophus and Pliolophus is presented by the absence of p 1 in the 

 former, which reduction of the number of the molar series to six, 

 M. Gervais regards as normal, and assigns as the chief generic di- 

 stinction from Lophiodon ; adopting in this respect the conclusions 

 of M. Pomel. The demonstration, which the rare perfection of the 

 skull and teeth of the Pliolophus vulpiceps from the London Clay 

 affords, of the retention of p 1 in the lower jaw, and consequently 

 of the typical dental formula, justifies the same generic distinction 

 of Pliolophus, as of Lophiodon proper, from the small Lophiodont 

 called Pachynolophus by Pomel. The generic distinction of Plio- 

 lophus from all previously known Lophiodonts is more decisively 

 established by the singular modifications of the grinding surface of 

 the lower molar teeth. 



This surface, in Pachynolophus, seems not to have been figured : 

 M. Gervais describes it, in the penultimate molar (m a), as present- 



