The Crustaceans of the Devonian are principally Eurypterids and Trilobites.  Some of the former attain gigantic dimensions, and the quarrymen in the Scotch Old Red give them the name of “seraphim” from their singular scale-like ornamentation.  The Trilobites, though still sufficiently abundant in some localites, have undergone a yet further diminution since the close of the Upper Silurian.  In both America and Europe quite a number of generic types have survived from the Silurian, but few or no new ones make their appearance during this period in either the Old World or the New.  The species, however, are distinct; and the principal forms belong to the genera Phacops (fig. 88, a, c, d), Homalonotus (fig. 88, b), Proetus, and Bronteus.  The species figured above under the name of Phacops latifrons (fig. 88, a), has an almost world-wide distribution, being found in the Devonian of Britain, Belgium, France, Germany, Russia, Spain, and South America; whilst its place is taken in North America by the closely-allied Phacops rana.  In addition to the Trilobites, the Devonian deposits have yielded the remains of a number of the minute Ostracoda, such as Entomis ("Cypridina"), Leperditia, &c., which sometimes occur in vast numbers, as in the so-called “Cypridina Slates” of the German Devonian.  There are also a few forms of Phyllopods (Estheria).  Taken as a whole, the Crustacean fauna of the Devonian period presents many alliances with that of the Upper Silurian, but has only slight relationships with that of the Lower Carboniferous.

Besides Crustaceans, we meet here for the first time with the remains of air-breathing Articulates, in the shape of Insects.  So far, these have only been obtained from the Devonian rocks of North America, and they indicate the existence of at least four generic types, all more or less allied to the existing May-flies (Ephemeridoe).  One of these interesting primitive insects, namely, Platephemera antiqua (fig. 89), appears to have measured five inches in expanse of wing; and another (Xelloneura antiquorum) has attached to its wing the remains of a “stridulating-organ” similar to that possessed by the modern Grasshoppers—­the instrument, as Principal Dawson remarks, of “the first music of living things that Geology as yet reveals to us.”

[Illustration:  Fig. 89.—­Wing of Platephemera antiqua Devonian, America. (After Dawson.)]

Amongst the Mollusca, the Devonian rocks have yielded a great number of the remains of Sea-mosses (Polyzoa).  Some of these belong to the ancient type Ptilodictya, which seems to disappear here, or to the allied Clathropora (fig. 90), with its fenestrated and reticulated fronds.  We meet also with the graceful and delicate stems of Ceriopora (fig. 91).