Lign. 199. Cephalaspis Lyellii. (1/4 nat. size.)
Devonian. Forfarshire.
Crushed specimen; seen from above.
Lign. 200. Cephalaspis Lyellii. (1/4 nat. size.)
Devonian. Glammis, Forfarshire.
Lateral view, showing the produced dorsal lobe of the tail.
Cephalaspis Lyellii. [Lign. 199], 200.—The most striking feature in the Ichthyolites of this genus is the enormous scutcheon, or buckler, which forms the head, and is prolonged posteriorly into two lateral horns or points; this part so closely resembles the cephalic shield of certain trilobites (see [Lign. 175]), that the first found specimens were supposed to be the remains of unknown crustaceans. The name Cephalaspis (buckler-head) is derived from this character. This remarkable appearance is occasioned by the intimate anchylosis of all the bones of the cranium. The body of these fishes is relatively smaller than the head; it has one dorsal fin, and terminates in a tapering tail, supporting a fin. There are two small eyes, placed towards the middle of the head. The body is covered with rhomboidal scales; and the head with discoidal scales, which are highly ornamented with radiated markings[544] (Ly. p. 344, fig. 396). There are four species of Cephalaspis at present known.
[544] Poiss. Foss. tom. ii. p. 135.
The other genera are equally unlike any recent types of the class of fishes. No perfect examples have been found, and some parts of their structure are still unknown; the annexed figures, [Lign. 201], have been drawn by Mr. Dinkel (the eminent artist employed by M. Agassiz), with scrupulous accuracy, no part being introduced which is not clearly demonstrated in some one specimen; and the form of the scutcheons is made out very distinctly, to aid the collector in discriminating the detached plates, which are the most common relics of these singular beings.
Pterichthys cornutus.[545] [Lign. 201], fig. 1.—This fish is distinguished by its two wing-like lateral appendages; whence the name of the genus (winged-fish). These processes, like the spines on the gill-covers of the common Bullhead (Cottus gobio), are weapons of defence. In some specimens they are extended at right angles to the body (Ly. p. 345, fig. 400).
[545] The first fish of this genus was discovered by Mr. Hugh Miller, in whose charming little work, "The Old Red Sandstone; or, New Walks in an Old Field," will be found a very graphic description of the Old Red fishes; I know not a more fascinating volume on any branch of British geology. Consult also Sir P. Egerton’s elaborate paper on the Pterichthys in the Geol. Journal, vol. iv. p. 302.
The head and anterior part of the body are covered with large angular tuberculated scutcheons. The under surface of the body is flat and protected by five plates, a quadrangular plate occupying the centre. The upper part of the body is convex; the form and disposition of the dorsal scutcheons are shown in [Lign. 201], fig. 1. There are two eyes, which are placed in front of the lateral spines: see fig. 1. The tail is of an angular form, and as long as the body; it is covered with scales, and considered by M. Agassiz to have been the only instrument of locomotion. The British species of this genus, of which ten are known, are all very small, varying in length from one to ten inches.