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Show 1887.] TELEOSTEAN GENUS RHACOLEPIS. 539 The eye has an ossified sclerotic capsule, and some of the soft parts of the fish are more or less indicated in nearly all the fossils. The gills are well seen when the opercular apparatus is partly removed, the lamellae being long and slender and reaching the hinder margin of the gill-cavity. The great muscles of the side of the trunk are also fossilized ; the successive myotomes, with their transverse muscular fibres, being especially distinct in one specimen in the Enniskillen Collection, which has already been referred to by Agassiz *. Specific Types. In his original notice of Rhacolepis (misprinted " Rhacolepis ") Agassiz recognized three distinct specific types, which he very briefly defined as separated by the form of the body and the characters of the posterior elements in the circumorbital ring. These, it appears, are also readily distinguished by the shape of the operculum, and perhaps some other features; and all the examples in the British Museum may be referred to one or other of the three forms. They received the names of R. buccalis, R. brama, and R. latus, and figures of each are given in our Plates. 1. RHACOLEPIS BUCCALIS. (Plate XLVI. figs. 2-7; Plate XLVII. figs. 1-3.) This is the smallest species, and comprises the fossil already mentioned as figured by Spix and Martius. It is of a very elongated shape, the greatest depth of the trunk being comprised about five and a half times in the total length. The two posterior c rcum-orbitals are elongated and approximately of equal size, and the length of the postero-inferior plate likewise much exceeds the depth. The vertical measurement of the operculum is much greater than its antero-posterior extent, the relative proportions being about 7 : 4. 2. RHACOLEPIS BRAMA. (Plate XLVI. fig. 1 ; Plate XLVII. fig. 4.) An indeterminable fragment of this species seems to have been originally noticed by Agassiz as Amblypterus olfersii2, and the latter specific name was thus substituted for brama in the " Synoptical Table" in the ' Rech. Poiss. Foss.' The body is somewhat less elongate than in R. buccalis, the greatest depth of the fossil shown in Plate X L V I . fig. 1 being contained about four and a half times in the total length. The two posterior circumorbitals are likewise much elongated, but the lower is narrower than the upper; and the postero-inferior plate has a deep triangular form. The length and breadth of the operculum are almost equal. 3. RHACOLEPIS LATUS. (Plate XLVII. fig. 5.) This is so called from the considerable depth of the body, as shown in the young individual figured. The two posterior circumorbitals 1 L. Agassiz, Rech. Poiss. Foss. vol. iv. pt. i. p. 2U3. 2 L. Agassiz, ibid. vol. ii. pt. i. p. 40. |