180 FOSSIL FISHES OF THE ENGLISH CHALK. 
by distant joits. They are all provided with upturned, slender denticles fixed in 
shallow sockets on their anterior face. On the few foremost rays these denticles 
are more or less closely clustered (Pl. XXXVIII, fig. 3 a), but on the other rays 
they are in two regular series (fig. 3b). At the base of the fin each ray tapers to 
a point and overlaps its own support, which is directly opposed to a corresponding 
neural or hemal arch. About two free supports can sometimes be observed, both 
above and below, immediately in advance of the fin. 
The scales are regularly arranged over the whole of the trunk, without enlarge- 
ment or reduction in any region. ‘They are thin, ovoid or cycloidal in shape, and 
deeply overlapping, and their inner surface is smooth, without ridge or boss (as 
proved by B.M. no. 49835). Their exposed portion is closely ornamented with 
elongated hollow tubercles or sharply pointed spinelets of ganoine, which are some- 
times finely striated at their base. In the anterior half of the abdominal region 
the tubercles on the scales are nearly uniform in size and form a regular 
ornamentation (Pl. XX XVIII, fig. 4 a), but behind the anterior dorsal fin nearly 
all the flank-scales of the fish exhibit an enlargement of the two or three denticles 
which form the median longitudinal row (fig. 4/)), and these denticles are less 
depressed than the others, so that they are usually broken away in the fossils and 
represented only by their hollow bases of attachment. The scales of Macropoma, 
indeed, are very rarely intact owing to the prominence of the ornament, which 
becomes more or less torn away in fractured specimens. The lateral line is not 
marked in any way except between the lobes of the caudal fin, where the scales of 
the series evidently traversed by it are thickened in the curious manner shown in 
Pl. XXXVIII, fig. 5. In the hinder fork of the thickening on each scale may be 
seen the broken hollow base of an enlarged median denticle. The manner in which 
these scales are pierced by the slime-canal is not clear. 
An elongated hard mass of cream-coloured phosphatic material is sometimes 
seen immediately above and behind the pelvic fins, and represents the infilled 
intestine of the fish. This body is round in section, with more or less rounded 
ends, and sometimes shows the mark of the spiral valve with which it was originally 
impressed. Similar coprolitic masses, of very fine and hard texture, are often 
found detached in the Chalk (Pl. XXXIX, figs. 1, 2), and some of them are so 
deeply marked by the spiral valve and wrinklings that they were at first mistaken 
for fossilised fir-cones.' They are commonly ascribed to Macropoma, and many of 
them are doubtless rightly so interpreted; but a large proportion of these 
coprolites are most likely referable to the Selachians and Chimeroids which 
abounded in the same sea. 
Portions of very small skeletons of Macropoma, with bones haying an appearance 
suggestive of immaturity, are sometimes met with in the English Chalk 
(Pl. XXXVITI, figs. 6, 7). So far as observable, the proportions of their head- 
1G. A. Mantell, Fossils of the South Downs (1822), pp. 108, 158, 310, pl. ix, figs. 3, 7, 10. 
