Among the anomalous forms of Old Red fishes not referable to Huxley’s Crossopterygii is the Pterichthys, of which five species have been found in the middle division of the Old Red of Scotland. Some writers have compared their shelly covering to that of Crustaceans, with which, however, they have no real affinity. The wing-like appendages, whence the genus is named, were first supposed by Hugh Miller to be paddles, like those of the turtle; and there can now be no doubt that they do really correspond with the pectoral fins.

The number of species of fish already obtained from the middle division of the Old Red Sandstone in Great Britain is about 70, and the principal genera, besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis, Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides.

Lower Old Red Sandstone.—The third or lowest division south of the Grampians consists of grey paving-stone and roofing-slate, with associated red and grey shales; these strata underlie a dense mass of conglomerate. In these grey beds several remarkable fish have been found of the genus named by Agassiz Cephalaspis, or “buckler-headed,” from the extraordinary shield which covers the head (see Fig. 502), and which has often been mistaken for that of a trilobite, such as Asaphus. A species of Pteraspis, of the same family, has also been found by the Reverend Hugh Mitchell in beds of corresponding age in Perthshire; and Mr. Powrie enumerates no less than five genera of the family Acanthodidæ, the spines, scales, and other remains of which have been detected in the grey flaggy sandstones.[[4]]

In the same formation at Carmylie, in Forfarshire, commonly known as the Arbroath paving-stone, fragments of a huge crustacean have been met with from time to time. They are called by the Scotch quarrymen the “Seraphim,” from the wing-like form and feather-like ornament of the thoracic appendage, the part most usually met with. Agassiz, having previously referred some of these fragments to the class of fishes, was the first to recognise their crustacean character, and, although at the time unable correctly to determine the true relation of the several parts, he figured the portions on which he founded his opinion, in the first plate of his “Poissons Fossiles du Vieux Grès Rouge.”

Carapace, showing the large sessile eyes at the anterior angles. The metastoma or post-oral plate (serving the office of a lower lip). Chelate appendages (antennules). First pair of simple palpi (antennæ). Second pair of simple palpi (mandibles). Third pair of simple palpi (first maxillæ). Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillæ. Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic segments. 7-12. Abdominal segments. 13. Telson, or tail-plate.)
  1. Carapace, showing the large sessile eyes at the anterior angles.
  2. The metastoma or post-oral plate (serving the office of a lower lip).
  3. Chelate appendages (antennules).
  4. First pair of simple palpi (antennæ).
  5. Second pair of simple palpi (mandibles).
  6. Third pair of simple palpi (first maxillæ).
  7. Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillæ.
  8. Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic segments. 7-12. Abdominal segments. 13. Telson, or tail-plate.)

A restoration in correct proportion to the size of the fragments of P. anglicus (Fig. 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire, would give us a creature measuring from five to six feet in length, and more than one foot across.

The largest crustaceans living at the present day are the Inachus Kaempferi, of De Haan, from Japan (a brachyurous or short-tailed crab), chiefly remarkable for the extraordinary length of its limbs; the fore-arm measuring four feet in length, and the others in proportion, so that it covers about 25 square feet of ground; and the Limulus Moluccanus, the great King Crab of China and the Eastern seas, which, when adult, measures 1½ foot across its carapace, and is three feet in length.