Pisces. | UPPER PALZOZOIC VERTEBRATA. 609 
dorsal; its width at base of the lobe slightly less than that of the pedicle of the tail at its origin, which is 
equal to one-fourth the space between the origin of the anal, and origin of the caudal; body prolonged 
nearly to the extremity of the upper lobe of the tail; all the fins with strong fuleral scales on their anterior 
edges. The fulcral scales on the upper lobe of the tail very large. Scales small, quadrate, nearly uniform over 
the whole body, the greater number of them rhombic, with the length and width nearly equal, but on the upper 
lobe of the tail becoming very acutely rhombic, with the length two or three times greater than the depth; 
those on the flanks near the head are oblong and nearly twice as deep as long; on all parts of the body five 
scales occupy a space nearly equal to one-fifth the depth of the body at that part ; surface of scales near and 
in front of the ventrals marked with about six or eight coarse, irregular, slightly flexuous longitudinal ridges ; 
on all the posterior parts of the body these ridges become almost imperceptible strize, even under a strong lens, 
and the surface is very minutely punctured; on all the middle of the body the scales average one line long; 
the average size of the specimens is usually about one foot five or six inches, and the depth near the middle 
from three and half to four inches. 
The bodies of the vertebree and spinous processes are strongly ossified, the latter inclining backwards at an 
angle of about 35° under the dorsal fin. The dorsal is as much in front of the anal as in the P. Humboldii, 
contrary to the supposition of M. Agassiz. The proportions of this large species are difficult to ascertain, 
owing to the great contortions of most of the specimens, permitted by the small size of the scales. Sir 
P. Egerton suggests, with great probability, that the anterior part of the body of this species, with the sculp- 
tured scales, was the foundation of Agassiz’ P. sculptus, the posterior half being his P. mandibularis. 
Position and Locality —Not very uncommon in the Permian marl-slate of East Thickley. 
Genus. ACROLEPIS (4g.) 
This genus differs from Pygopterus principally in the shorter anal fin, the dorsal being a little more 
anterior in position, and the scales being more strongly keeled and suleated diagonally ; but in size, form, and 
general characters, as far as known, they are so identical, that I greatly doubt the propriety of separating them. 
The genus is found in the carboniferous and Permian rocks. 
Acroteris Hopxriyst (M°Coy). Pl. 3. G. fig. 10. (lower figure). 
Syn. and Ref. = Holoptychius Hopkinsi McCoy, Ann. Nat. Hist. 2nd Series, Vol. II. 
Desc. —Scales elongate, narrow, elliptically pointed, very thick, convex ; exposed portion strongly polished, 
covered with numerous thick, rounded, slightly flexuous, anastomosing, longitudinal ridges; concealed smooth 
portion very large, deeply bifureated, generally seeming to be bent laterally at a considerable angle with the 
exposed portion, from the extension of the posterior margin being much greater than that of the superior 
margin. Average length of polished external portion six lines, width three lines. 
The thick narrow form and strong longitudinal ridges of the surface distinguish this species easily from 
its congeners. The specimens described were collected from the Derbyshire limestone by W. Hopkins, Esq., 
who presented them, with a suite of fossils from that district, to the University Collection. I have great pleasure 
in dedicating it to one who, as a mathematician, and as an observer in the field, has so materially advanced the 
science of Geology. I originally described this species under the genus Holoptychius, but the more perfect 
examples I have since seen induce me to place it in Acrolepis. Just as these sheets were going through the 
press, I have seen at the British Museum the specimen figured by M. de Koninck (Foss. Bel. t. 55. f. 5a) as 
undetermined fish-scales, and I find it belongs to the present species. 
Position and Locality.—It abounds in some parts of the black impure beds intercalated between the carbo- 
niferous limestone and overlying shale of Derbyshire. 
Explanation of Figures.—Plate 3. G. fig. 10 (lower figure), portions of two scales, natural size. 
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