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 s'lde 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 Rkacolepis (misprinted " Phacolepis ") 

 Agassiz recognized three distinct specific types, which he very briefly 

 defined as separated by the form of the body and the characters o'f 

 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 XL VI. 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 crcum- 

 orbitals are elongated and approximately of equal size, and the length 

 of the postero-mferior plate likewise much exceeds the depth. Tlie 

 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 indeterniinable fragment of this species seems to have been 

 origmally noticed by Agassiz as Amhlypterus ol/ersii\ and the 

 latter specific name was thus substituted for brama in the "Synop- 

 tical Table" in the 'Rech. Poiss. Foss.' The body is sonuwhat 

 less elongate than in R. buccalis, the greatest depth of the fossil 

 shown m Plate XLVI. fig. 1 being contained about four and a half 

 times m 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. Tlie 

 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 

 ^ L. Agassiz, Eech. Poiss. Foss. vol. iy. pt. i. p. 293. 

 ^ L. Agassiz, ibid. vol. ii. pt. i. p. 40. 



