Geological Society. 225 



because it is to those families only of mammifers that the fossils 

 have been considered by anatomists to belong ; and to the interior 

 surface of the ja\v, as the exterior is not exhibited in any of the fossil 

 specimens. 



1. In the general outline of the jaws, more especially in that of 

 the Didelphys (Phascolotherium) Bucklandii, the author states, there 

 is a very close resemblance to the jaw in recent insectivora and 

 insectivorous marsupials ; but he observes, that with respect to the 

 uniform curvature along the inferior margin, Cuvier has adduced 

 the same structure as distinctive of the Monitors, Iguanas, and other 

 true saurian reptiles, so that whatever support these modifications of 

 structure may give to the question respecting the marsupial nature 

 of the Stonesfield fossils, as compared with other groups of mammals, 

 they do not affect the previous question of their mammiferous na- 

 ture, as compared with reptiles and fishes. The fossil jaws, Mr. 

 Ogilby says, agree with those of mammals, and differ from those of 

 all recent reptiles, in not being prolonged backward behind the 

 articulating condyle ; a character in conjunction with the former 

 relation which would be, in this author's opinion, well nigh incon- 

 trovertible, if it were absolutely exclusive ; but the extinct saurians, 

 the Pterodactyles, Ichthyosauri, and Plesiosauri, cotemporaries of the 

 Stonesfield fossils, differ from their recent congeners in this respect 

 and agree with mammals. Mr. Ogilby is of opinion that the con- 

 dyle is round both in D. Prevostii and D. Bucklandii, and is there- 

 fore a ver)^ strong point in favour of the mammiferous nature 

 of the jaws. The angular process, he says, is distinct in one speci- 

 men of D. Prevostii, and, though broken cff in the other, has left a 

 well-defined impression ; but that it agrees in position with the insec- 

 tivora, and not the marsupialia, being situated in the plane passing 

 through the coronoid process and the ramus of the jaw. In the 

 D. Bucklandii, he conceives, the process is entirely wanting ; but 

 that there is a slight longitudinal ridge partially broken, which 

 might be mistaken for it, though placed at a considerable distance up 

 the jaw, or nearly on a level with the condyle, and not at the 

 inferior angular rim of the jaw. He is therefore of opinion that the 

 D. Bucklandii cannot be properly associated either with the marsu- 

 pial or insectivorous mammals. The composition of the teeth, he 

 conceives, cannot be advanced successfully against the mammiferous 

 nature of the fossils, because animal matter preponderates over 

 mineral in the teeth of the great majority of the Insectivorous Cheir- 

 optera, as well as in those of the Myrmecobius, and other small marsu- 

 j)ials. In the jaw of the D. Prevostii, Mr. Ogilby caimot perceive 

 any ajjpcarance of a dcntary canal, the fangs of the teeth, in his 

 opinion, almost reaciiing tiie hiferior margin of the jaw, and being 

 implanted comjiletely in the bone ; but in the J). Bucklandii, he has 

 observed, towards the anterior extremity of the jaw, a hollow 

 space filled witii foreign matter, and very like a dcntary canal. The 

 double fangs of the teeth of D. Prevostii, and probably of D. Buck- 

 landii, he says, are strong points of agreement between the fossils 

 and uiamrnifers in general ; but that double roots necessarily indi- 



Phil. Mac/, y. y. Vol. 14. No. 88. March 1839. Q 



