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I.—Gems from Private Collections, No. 2
On the Genus Æchmodus from the Lias of Lyme Regis, Dorsetshire.
Published online by Cambridge University Press: 01 May 2009
Extract
Among the many fossil remains of fishes obtained from the rich Liassic deposits near Lyme Regis, species of the genera Dapedius, Æchmodus, Semionotus, and Pholidophorus are most frequently found in a good state of preservation. These Lepidoid genera are chiefly Liassic, two of the more long-bodied froms, Pholidophorus and Semionotus, having species which occur in the Purbeck (Phol. ornatus) and the Chalk Strata (Sem. Bergeri). Lepidotus, another Lepidoid, ranges from the Lias through the intermediate Oolite and Wealden strata into the Chalk, and according to Sir P. Egerton, “the genus Lepidotus has the most extensive geographical range of any genus of fossil fish.’ With the exception of Lepidotus, Prof. Owen classes the above genera under the Dapedoid family of the order Lepidogamoidei, the genus Dapedius (D. politus) first noticed by the late Sir H. De la Beche forming the type of the family. All the genera were arranged by Sir P. Egerton under the Lepidoid family of the order Ganoidei, and Mons. Pictet placed them in his second family of Rhombiferous Ganoids—the Lepidosteidœ, and under the second tribe of that family—the homocercal Lepidosteidœ, (Lepidoides Homocerques, Ag.), Which he further subdivided into sections, some of which included the above-mentioned genera—viz.
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References
page 337 note 1 Palæontology, 1861, 2nd Edit., p. 166.Google Scholar
page 337 note 2 Geol. Trans., 2nd Series, Vol. i. pl. 6, fig. 1–4.Google Scholar
page 337 note 3 Traité de Paléontologie, 2nd Edit., 1854, Vol. ii., p. 157.Google Scholar
page 338 note 1 Among the many examples of Fossil Fishes belonging to this genus which are preserved in the National Museum, Mr. W. Davies (to whom I am exceedingly indebted for much valuable assistance in drawing up this paper, and whose knowledge of Fossil Ichthyology is very extensive) assures me that no trace of a decided bony column exists in any well authenticated species of Lepidotus with the exception of Lepidotus serrulatus from the Lias of Barrow, and L. faimbriatus from Lyme Regis, both of which species, from other peculiarities, may eventually be found to form a sub-genus. I am therefore led to conclude that the complete ossification of the notochord is not a characteristic point in Lepidotus, as stated by M. Pictet. Prof. Pictet says, op. cit. p. 161, Le Lepidotus fimbriatus, Ag. est une espèce dont la position générique est encore douteuse. Les écailles ont une fine dentelure sur leur bord. Le L. serrulatus, Ag. a des rapports avec le L. gigas, mais en diffère, ainsi que de presque tous ses congénères, par ses écailles qui sont plus étroites vers le bord ventral.
page 338 note 2 Mem. Geol. Survey, 1861. Decade x. p. 28.Google Scholar
page 338 note 3 Quart. Journ. Geol. Soc., Vol. ix. p. 276.Google Scholar
page 338 note 4 Egerton, Quart. Journ. Geol. Soc., 1854, Vol. x. p. 367.Google Scholar
page 340 note 1 Quart. Journ. Geol. Soc., Vol. ix. p. 275.Google Scholar
page 340 note 2 Ibid. p. 352.
page 340 note 3 With regard to these terms, see the remarks by Prof. Huxley, Quart. Journ. Mic. Science, Oct. 1868, and Mem. Geol. Survey, Decade x. to p. 3, where he states “that the so-called ‘homocercal’ Teleostei of the present epoch are in reality excessively heterocercal; but the word ‘homocercal’ is now so generally understood to signify a tail like that of most existing Teleostei, that I prefer to employ Prof. M'Coy's term ‘diphycercal’ for truly homocercal tails.”
In alluding to these structures. Prof. Owen writes:-“The shape of the caudal fin varies much in fishes, according to the kind and degree of motion required: in the imprisoned embryo, in the long and slender undulating eel, in the sluggish Lepidosiren, the vertebræ continue to the end of the body in a straight line, distinct and decreasing to a point; and the tail is bordered above and below by a vertical fold of skin; terminating either in a point or obtusely. Such fold or fin is symmetrical, but not ‘homocercal.’ The vertical folds deepen; at first equally, forming a terminal lobe; then excessively, in the lower or hæmal fold, with the developement therein of rays, and with an upward or neural inclination of the Exporting vertebrae. Shorter rays are developed in the shallower neural fold, which terminates at the pointed end of the vertebral series. The anterior rays of the hæmal fold, which are the longest, form a second point. The tail is thus bifurcate, but unsymmetrical; and this stage of the developement is termed the ‘heterocercal’ one. It was the fashion of tail which prevailed in fishes throughout the palæozoic and triassic periods. In some oolitic fishes, first is observed such a lengthening of the dermoneurals of the tail, with such a shortening and running together of the terminal vertebræ, and such a proportion of the dermohæmals, as leads to an equal-lobed candal fin, which has been termed ‘homocercal;’ but as his only symmetrical in contour, and remains more or less unsymmetrical in its framework, I term it ‘homocercoid.’ The ganoid fishes of the mesozoic periods manifest several interesting gradations of this transitional state from the hetero- to the true, homo-cereal form, each step being a permanent character of the extinct species presenting it. —Comparative Anatomy and Physiology of Vertebrates, 1866, Vol. i, p. 253.