THE OLD RED SANDSTONE. 157 
compared with the size of the body, was covered with bony 
plates, roughened after a pattern somewhat different from 
that of the scales, being tubercled rather than ridged; but 
the tubercles present a confluent appearance, just as chains 
of hills may be described as confluent, the base of one hill 
running into the base of another. The operculum seems to 
have been covered by one entire plate—a peculiarity ob- 
servable, as has been remarked, among some of the ichthy- 
olites of the Lower Old Red Sandstone, such as the Diplop- 
terus, Dipterus, and Osteolepis. And it, too, has its fields of 
tubercles, and its smooth marginal selvedge, or border, on 
which the lower edges of the upper occipital plates seem to 
have rested, just as, in the roof of a slated building, part of 
the lower tier of slates is overtopped ana covered by the tier 
above. The scales towards the tail suddenly diminish at 
the ventral fins to about one fourth the size of those on the 
upper part of the body; the fins themselves are covered at 
their bases, which seem to have been thick and fleshy like 
the base of the pectoral fin in the cod or haddock, with scales 
still more minute ; and from the scaly base the rays diverge 
like the radii of a circle, and terminate in a semicircular out- 
line. The ventrals are placed nearer the tail, says Agassiz, 
than in any other ganoid fish. (See Plate XIIL., fig. 2.) 
But no such description can communicate an adequate con- 
ception to the reader of the strikingly picturesque appearance 
of the Holoptychius, as shown in Mr. Noble’s splendid speci- 
men. There is a general massiveness about the separate 
portions of the creature, that imparts ideas of the gigantic, 
independently of its bulk as a whole; just as a building of 
moderate size, when composed of very ponderous stones, has 
a more imposing effect than much larger buildings in which 
the stones are smaller. The body measures a foot across, by 
16 * 
