Discussion.
The President differed from the author as to the conclusions he drew from the structure of the teeth. The teeth of existing Crocodilia had been but imperfectly described, and he thought he could point out among existing Crocodiles teeth bearing the character which the author regarded as Lacertilian. He agreed with Prof. Owen in regarding Dakosaurus as Crocodilian rather than Dinosaurian or Lacertilian.
Mr. Wood Mason had seen in the Gavial of the Ganges, and in the teeth of Teleosaurians from the neighbourhood of Oxford, the same crenulations and compression, which he regarded as indicative of a Lacertilian character.
4. On the Anatomy of the Test of Amphidetus (Echinocardium) Virginianus, Forbes ; and on the Genus Breynia. By P. Martin Duncan, M.B., F.R.S., Sec. G.S., &c.
(This paper was withdrawn by permission of the Council.)
[Abstract.]
After a careful examination of the Miocene Amphidetus from the Virginian Tertiaries, the author regarded the recent species of the genus from the European and Australian seas as a group of very closely allied forms. The Crag specimen of A. cordatus, described by Forbes, could not be found ; but the examination of a series of recent specimens decided that they were not specifically different from the Miocene form.
The unusual form of the ambulacral spaces, the nature of the fascicle crossing them, and the resulting absence (more or less) of pores within the fascicle were asserted to be of third-rate value as regards structural importance ; and the author did not consider that the genera Echinocardium, Breynia, Lovenia, &c. had a common origin, or that there was a close genetic relationship between them, because they had this fasciolar structure. He considered the fascicle to be an appendage to several generic groups which were distinctly separated by other structural distinctions. An examination of the Nummulitic Breynioe in the Society's collection satisfied Dr. Duncan that there were only race characters separating them from Breynia Australiensis — a recent Echinoderm. The persistence of these species, widely distributed, and of great geological age, was very remarkable.
Discussion.
The President regretted, with the author, the prevailing custom of determining species as much by their geological position as by their structural affinities. He thought it was necessary to have a knowledge of living forms, in order to estimate correctly the value of the characters of extinct species. He considered that the presence
