696 STRATIGRAPHICAL GEOLOGY.  [Boox VI. 
hippurite, and a brachiopod; but is now regarded as a rugose coral 
possessing an opercular lid. The Plewrodictyum problematicum, a 
well-known form of the Lower Devonian beds, is now classed with 
the Favositid# among the perforate corals. Numerous forms of the 
puzzling genus Stromatopora occur in some of the limestones ; and the 
curious Heceptaculites, already (p. 663) referred to, is a well-known 
Devonian fossil. The corals of the Devonian seas were both abundant 
in individuals and varied in their specific and generic range. Not 
a single species is common either to the Silurian system below or 
the Carboniferous above. Among the rugose forms the genera 
Cyathophyllum, Acervularia, and Cystiphyllum are characteristic. 
The tabulate kinds belonged chiefly to the two important genera of 
Favosites and Alveolites. Of the echinoderms by far the most 
abundant representatives are crinoids, which occur in great profusion 
in the limestones, sometimes forming entire beds of rock. They 
belong chiefly to the two families of Cyathocrinide, simple 
pedunculate forms with five branching arms, and the Cupresso- 
crinidz, haying five arms which when folded up form a pentagonal 
pyramid the accurate fitting of which recalls the ambulacra of sea- 
urchins. ‘The Cystideans appear to have died out in the Devonian 
period. True star-fishes also occur (Helianthaster, Astropecten, Coelaster). 
The known crustacean fauna of the Devonian period indicates a 
striking diminution both in number of individuals and of species of 
trilobites (Fig. 329). Most of the genera so abundant and character- 
istic among the Silurian rocks are now absent, the most frequent 
Devonian forms being species of Phacops, Homalonotus, Dalmanites, 
and Bronteus. ‘The ostracods are chiefly represented by the genus 
Entomis (Cypridina), which occurs in enormous numbers in some 
Upper Devonian shales (“ Cypridinen-schiefer”), The phyllopods 
and eurypterids occur chiefly in the Old Red Sandstone, and are 
noticed on p. 710 (Fig. 329, d). Altogether 45 genera and 290 
species of Devonian crustacea are known. 
Among the mollusca of the Devonian rocks remains of the 
pteropod Tentaculites are not uncommon. The brachiopods now 
reached perhaps their maximum development, whether as regards 
individual abundance or number of specific and generic forms : 
no fewer than 61 genera and 1100 species having been described, 
They compose three-fourths of the known Devonian fauna. While 
all the families of the class are represented, the most abundant are 
the Spiriferide, including the genera Spirifera, Oyrtia, Athyris 
(Spirigera), Uncites, and Atrypa, and the Rhynchonellide, Lthyn- 
chonella, Camarophoria, and Pentamerus. The Strophomenids or 
Orthids, so abundant in the Silurian rocks, are now represented b 
a waning number of forms, including the genera Orthis, Strophomena 
Streptorhynchus, and Leptena. The Productids made their appear- 
ance in Silurian times, but were more abundant in the Devonian 
seas, where their most frequent genera were Chonetes and Productus 
both of which attained their maximum development in the Carbon- 
iferous period, One of the most characteristic and largest Devonian 
