526 Correspondence — Mr. J. J. MurpJnj — Prof. E. D. Cope. 



the roots of mountain chains forming ridge-shaped projections on 

 the lower surface of the solid crust. 



Perhaps it is not generally known how nearly this resembles a 

 theory propounded by the late Mr. Hopkins of Cambridge thirty 

 or forty years ago. He maintained that the earth is solid at the 

 surface from cooling, and at the centre from pressure; that the 

 solid centre is for the most part continuous with the solid crust; but 

 that in volcanic regions there are subterranean lakes of molten matter 

 between the two. 



I speak with no authority, but I think it most probable that the 

 earth is solid throughout, with the possible exception of small and 

 perhaps only temporary reservoirs of lava. The fact that the lava 

 in neighbouring craters often stands permanently at different levels, 

 proves that they cannot be in communication with a common 

 reservoir ; and the tremulousness of the earth's surface, which the 

 microphone reveals, seems to me to prove only that the materials 

 composing it are elastic and slightly flexible. 



Belfast, 9th Sept. 1885. JoSEPH JoHN McJRPHY. 



MR. LYDEKKER ON ESTHONYX. 



Sir, — The article in your August number by Mr. Lydekker on 

 the identity of Esihonyx, Cope, with Flatyclicerops, Charlesworth, 

 excited my interest, and requires a few words of comment. It is 

 of the greatest importance to determine, if possible, the identity of 

 generic and specific forms in widely separated localities during past 

 geological ages. This has occasionally been successfully accomplished, 

 as, for instance, the determination of Hyrachyus, by Gaudry, and of 

 Oxycena, by Filhol, in France. In other cases discovery of missing 

 parts has shown that such supposed identification were premature. 

 Thus, I have been compelled to recede from some identifications of 

 American with European Lemuroids. 



After an examination of Prof. Owen's figures and description of 

 Miolophus pJaniceps^ cited by Mr. Lydekker, I find that the 

 identity of Esthonyx with Miolophus is extremely improbable, and 

 could as well be asserted of at least one other genus. Indeed, there 

 is nothing in the technical characters of the superior molars to pre- 

 vent the identification of Miolophus with Chriacus, Mioclaenus or 

 Deltatherium, genera which only differ from each other in the 

 characters of the superior and inferior premolars and inferior molars. 

 But Esthonyx differs still more from the normal types in its very 

 peculiar incisors. In order that Miolophus should be identified with 

 E^ithonyx under these circumstances, some evidence as to the 

 characters of its incisors should be obtained, which is not the case as 

 yet. Mr. Lydekker appears to attach some importance to a space 

 behind p.m. 3. This space in the specimen of Esthonyx Burmeis- 

 teri figui'ed by me, may be due to accident, as the maxillary bone is 

 in bad condition, and a fissure traversed the first true inferior molar. 

 There is also a good reason for suspecting that the genera in question 

 are not identical. This is the presence of a loop-like inner posterior 

 1 Tlatj/chcerops Biehardsoni, teste Lydekker. 



