[Illustration: Fig. 88. a b, _Spirorbis laxus_, enlarged, Upper Silurian, America; c, _Spirorbis spinulifera_, of the natural size and enlarged, Devonian, Canada. (After Hall and the Author.)] [Illustration: Fig. 88.--Devonian Trilobites; a, _Phacops latifrons_, Devonian of Britain, the Continent of Europe, and South America; b, _Homalonotus armatus_, Europe; c, _Phacops (Trimerocephalus) loevis_, Europe; d, Head-shield of _Phacops (Portlockia) granulatus_, Europe. (After Salter and Burmeister.)] 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.)]