DESCRIPTION OF A SECOND GANOID FISH. 



153 



bifurcation which is almost as symmetrical as the true 

 homocercal forms ; the rays on the lower margin of the true 

 tail lobe form a narrow inconspicuous fringe. All the rays 

 of caudal extremity are extremely fine, about 21 in the space 

 of 12 lines, those of the sub-caudal fin are crossed repeatedly 

 with curved, raised lines, composed of short, oblique ridges as 

 in dorsal fin. 



There are about 64 rows of small rhomboid scales or plates 

 longitudinally arranged in an inclined and slightly curved 

 dorso-ventral series. The transverse series varies considerably 

 in number and depth ; there being about 18 in oblique trans- 

 verse series near peduncle, and about 35 near the shoulder ; 

 the scales are greatest in depth towards the lateral line, 

 diminishing greatly as they approach dorsal and ventral 

 margins ; the "base of each scale runs parallel nearly with 

 others of the longitudinal series, and in a general view seem 

 to divide the surface into fine parallel rows, which cut the 

 more or less curved oblique transverse series into an oblique 

 rhomboid pattern. 



Each scale or plate is ornamented on the upper surface 

 with the peculiar furcated raised linear ridges of the genus 

 Acrolepis ; these forked ridges seem to radiate from a point 

 at the lower posterior angle and fork once or twice, upward 

 and forward, in the direction of the anterior upper angle. 

 This furcation reminds one of the forking neuration on the 

 lobes of many species of ferns, and is very remarkable. 

 Fortunately a portion of the fossil shows the under surface, 

 and each scale of these series is characterised by a small 

 tooth-like projection in the centre of each upper margin, with 

 a corresponding raised socket on lower surface half the depth 

 of scale, into which the tooth of each scale respectively of the 

 lower series scale fits closely. Both socket and tooth have an 

 angle corresponding to the direction of the transverse serial 

 lines; the ornamentation of the scales forming the lateral 

 line differs from all the others, as the fine furcate lines 

 radiate more nearly from the centre of posterior margin, half 

 of which curve upwards, and the other half downwards. As 

 a rule the linear ridges are fewer in number relative to size of 

 scale in the lateral line series. 



This fine specimen was obtained from the Lower Mesozoic 

 Sandstones near Tinder-Box Bay.* A group probably 

 belonging nearly to the same geological horizon as the 

 Knocklofty sandstones, Hobart, from which was obtained the 



' In p. 149, "Johnston's Geology of Tasmania," these sandstones are described as 

 "a series of reddish or yellow micaceous sandstones, sometimes of great thickness, 

 often occur overlying the uppermost beds of the Upper Pakeozoic Mudstones as at 

 Tinder-Box Bay, Huon-road, and Waterworks Valley, near Hobart, with indistinct 

 plant impression and silicified trunks of conifer. These sandstones apparently lie 

 conformably and without stratigraphic break upon the uppermost beds . ot tne 

 Upper Marine series (Up. Pal.). Their position is assigned provisionally at the Base 

 of the Mesozoic rocks of Tasmania."— (Johnston. ) 



