208 FOSSIL JAWS FROM STONESFIELD. 



" A paper was afterwards read, entitled ' Observations on the structure 

 and relations of the presumed marsupial remains from the Stonesfield 

 oolite,' by William Ogilby, Esq., F.G.S. 



" These observations are intended by the author to embody only the most 

 prominent characters of the fossils, and those essential points of structure in 

 which they are necessarily related to the class of mammifers or of reptiles 

 respectively. For the sake of putting the several points clearly and impar- 

 tially, he arranged his observations under the two following heads : — 



" 1 . The relations of agreement which subsist between the fossils in ques- 

 tion and the corresponding bones of recent marsupials and Insectivora. 



"2. The characters in which the fossils differ from those families. Mr. 

 Ogilby confined his remarks to Marsupialia and Insectivora, 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 jaw, 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 marsu- 

 pials ; 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 

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

 says, agree with those of mammals, and differ from those of all recent rep- 

 tiles, in not being prolonged backward behind the articulating condyle ; a 

 character in conjunction with the former relation, which would be, in the 

 author's opinion, well nigh incontrovertible, 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 condyle is round both in Did. Prevostii and Did. Bucklandii, and is 

 therefore a very strong point in favour of the mammiferous nature of the 

 jaws. The angular process, he says, is distinct in one specimen of Did. 

 Prevostii, and, though broken off in the other, has left a well-defined im- 

 pression ; but that it agrees in position with the Insectivora, and not the 

 Marsupialia, being situated in the plane passing through the coronoid pro- 

 cess and the ramus of the jaw. In the Did. 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 consi- 

 derable 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 

 Did. Bucklandii cannot be properly associated with either the marsupial or 

 insectivorous mammals. The composition of the teeth, he conceives, can- 

 not 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 Cheiroptera, as well as in those of the Myrme- 

 cobius, and other small marsupials. In the jaw of the Did. Prevostii Mr. 

 Ogilby cannot perceive any appearance of a dentary canal, the fangs of the 

 teeth, in his opinion, almost reaching the inferior margin of the jaw, and 

 being implanted completely in the bone ; but in the Did. Bucklandii he 

 has observed, towards the anterior extremity of the jaw, a hollow space fill- 

 ed with foreign matter, and very like a dentary canal. The double fangs 

 of the teeth of Did. Prevostii, and probably of Did. Bucklandii, he says, are 

 strong points of agreement between the fossils and mammifers in general ; 



