Abstract
LONDON
Geological Society, March 6.-Prof. Duncan, F.R.S., vice-president, in the chair.-(i.) “Prognathodus Guntheri (Egerton), a new genus of fossil Fish from the Lias of Lyme Regis.” By Sir P. de M. Grey-Egerton, Bart, M.P., F.R.S. In this paper the author described a new form of fossil fish, having a broad premaxillary plate somewhat resembling the incisor tooth of a gigantic Rodent, a single auxiliary plate like that of Callorhynchus, and a mandibular dental apparatus closely resembling that of Cochliodus. For this form he proposed the establishment of the new genus Prognathodus, and named the species P. Guntheri. Ischyodus 'Johnsoni, Agassiz, also probably belongs to this genus, as it agrees with P. Guntheri in the characters of the premaxillary teeth. The author was doubtful as to the exact position of this genus, which had a head extended in a horizontal instead of a vertical plane, suggesting a resemblance to Zygoena, but covered with hard plates like the head of a sturgeon, and exhibited in the dental apparatus the curious combination indicated above.-Dr. Giinther pointed out the interest attaching to the dentition of this fossil fish as proving the connection between the Ganoid and Chimaeroid forms. The existence of three teeth instead of one on each side of the jaw, as in Ceratodus and others, presented in it a generic character; but the type was still the same. Mr. Etheridge made some observations as to the horizon in the Lias in which these fossil fishes occurred. He believed that nine out of ten of the Lower Lias Species came out of the upper part of the Bucklandi limestone series. Sir P. Egerton corroborated Mr. Etheridge's views as to the localisation of species of fish, and agreed with him as to the importance of recording the exact position of all such fossils.-(2.) "On two specimens of Ischyodus, from the Lias of Lyme Regis.” By Sir P. de M. Grey-Egerton, Bart., M.P., F.R.S. In this paper the author noticed a new example of the greatly developed rostrum of a male Chimeroid, an inch shorter, more slender, and more attenuated at the apex, than that of Ischyodus orthorhinus Egerton, having a projecting median rib along the upper surface, and the tubercles of the lower part smaller and fewer than in I. orthorhinus. For this form the author proposed the name of /. leptorhinus. Also a dorsal fin-spine, with the cartilages to which it was articulated, showing the mechanism of its attachment very clearly. This spine differs from that of 1. orthorhinus in being straighter and smoother, and having fewer and smaller tubercles. The author regarded it as probably belonging to I. leptorhinus-(3.) “How the Parallel Roads of Glen Roy were formed.” By Prof. James Nicol, F.G.S. In this paper the author endeavoured to explain, in accordance with the marine theory of the origin of the Parallel Roads of Glen Roy, the coincidence of the level of these terraces with that of the different cols, and also how the same sea could have produced terraces at different levels in different valleys. He assumed that during the gradual elevation of the land, the gradual closing of the straits between its separate masses by the elevation of the cols above the surface would, by checking the eastward flow of the tidal current, cause the sea-level in the western bays to remain stationary relatively to the rising land; and during this period the marine erosion would take place along a line corresponding in level to the col. Hence, in Glen Gloy, which has only one col, the highest in the system, the highest road only was formed; and Glen Gloy remained unaffected by the stoppage of those cols which produced three roads at lower levels in Glen Roy, the lowest of them also extending round Glen Spean. Professor Ramsay entered into the history of the theories for accounting for the terraces, the first of which, that of Professor Agassiz (in 1840), accounted for them by a great glacier damming up the valley, and from time to time declining in height. The glacial theory, on which this view rested, had to some extent been doubted, but eventually had been almost universally accepted even by its first opponents. He next cited the works of the late Mr. Robert Chambers as to the existence of old sea-margins, pointing to su gradual sinking of the sea or a rising of the land. There could be little doubt that a great part of Scotland and of the northern part of England, had been at one-time covered with glaciers, as had also been the case in other parts of Europe. Unless the whole country had been submerged, and then came up again by a succession of jerks, it seemed impossible that such terraces could have been formed by the sea and still have remained in existence. If, however, there had been great oscillations in temperature, it seemed possible that during the decline of some transverse glacier the varying levels of the lake might have left terraces, traces of which might still be preserved. Mr. Gwyn Jeffreys renewed his protest against regarding these beds as marine unless marine remains were found in them. In Prof. Nicol's former paper, mention, however, had been made of rolled boulders. These occurred at Glasgow, and elsewhere, covered with Balani. As, however, no marine remains had been found in Glen Roy, he adopted the "freshwater theory. Mr. Evans regretted that no one else was present who would in any degree advocate the author's views. He pointed out that if the surface of the rocks below the detritus in Glen Roy was glaciated, the probability was in favour of the superficial drift being of marine rather than of subaenal origin. He much doubted whether Ben Nevis, or any of the mountains of the district, offered a sufficient gathering-ground for any such glacier as that supposed in the freshwater theory, assuming the climate to have been such as would have admitted of a large lake in Glen Roy. He suggested the possibility of the openings through which the sea would gain access to the district having at the time of the last submergence been to some extent choked with ice, which thus checked the tidal action inland from the present coast; and thought that possibly both glaciers and the sea had together contributed towards the formation of the terraces. These, he observed, were by no'means confined to Glen Roy itself, but were to be seen on a large scale, and at a lower level in the valley of the Speam, if not elsewhere.
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Societies and Academies . Nature 5, 435–436 (1872). https://doi.org/10.1038/005435a0
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DOI: https://doi.org/10.1038/005435a0