190 Messrs. Hancock & Atthey on Diptenisanc^Ctenodus, XXIII. — A feio Remarks on Dipterus and Ctenodus, and on their Relationshij? to Ceratodus Forsteri, Krefft. By Albany Hancock, F.L.S., and Thomas Atthey. [Plates XIII. & XIV.] In our paper on Ctenodus^ published in the 'Annals ' in Feb-rnaiy 1868*, while recognizing the close connexion between this genus and Dipterus^ we deemed it prudent to keep tlie two forms asunder, for certain reasons therein expressed ; and the time that has elapsed since then has only tended to con-firm us in this opinion. Among other characters that in-fluenced us, stress Avas laid on the differences in the scales in the two genera ; and allusion was made to the fact that the Dipteri are all small fishes, and that the Gtenodi^ on the con-trary are, with one exception, all of considerable size. In the paper alluded to, seven species were described, six of which were new. Three of the seven (namely, G. cristatus^ C. tuherculatus,sind C.corrugatus) cannot have been less than five or six feet in length. We originally estimated the length at four or five feet ; but as larger specimens have since come to hand, we now think that that estimate was too low. Three others (namely, C. ohh'quusj G. imhricatus, and G. ellijJficiis) were pro-baljly upwards of three feet long. G. eJegans is quite small. The latter is the only species of which an entire specimen has occurred ; and though much crushed and disturbed, its dimen-sio7is can be determined with sufficient accuracy : it measures only three inches in length, but, judging from the size of de-tached dental plates, it probably reaches sometimes nearly twice that length. The Dipteri are usually about five or six inches long, and apparently never much exceed that length. The scales are very different in the two forms. While in Dipterus they are circular and truly cycloidal, in Gtenodus they (PI. XIII. fig. 3) are elongated and parallelogrammatic, with the posterior end well rounded, and the sides nearly parallel or a little hollowed or concave ; they are in length nearly twice their width, and, though imbricated, can scarcely be called truly cycloidal : they are delicate and large for the size of the fish, and are longitudinally ridged or grooved ; the ridges, becoming curved and nodose, form a sort of rosette in the centre of the exposed imbricated portion. This is very different from the ornamentation of the scales of Dipterus^ which are either irregularly tuberculated, the tubercles being * "Notes on various Species of Ctenodus from the Nortlmmberland Coal-Field," Ann. Nat. Tiist. ser. 4. vol. i. p. 77.
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