166 GEOLOGICAL DISTRIBUTION. 



theus, Arietites, Harpoceras, ^Egoceras, Stephanoceras, Lytoceras, 

 Phylloceras), and beletfinite ; but representatives of groups that 

 appear to have been closely related to the modern calamary (Belo- 

 teuthis, Belemnosepia, Teuthopsis) are not exactly wanting. The 

 lamcllibranchs and gasteropods comprise a most varied assemblage 

 of forms, many of them but barely distinguishable from individual 

 forms living at the present day, and by their great numerical de- 

 velopment give a generally modern aspect to the fauna. Of the 

 former the modern families Ostreidoa, Limidse, Mytilidae, Astartidse, 

 Lucinidas, and Cardiida3 are remarkable for their profuse develop- 

 ment, and scarcely less so the now nearly extinct Trigoniadaa and 

 Pholadomyidffi ; of the latter, the more important families are still 

 the non-siphonated ones (Pleurotomariida3, Naticidse, TrochidaB, 

 Actseonidae) ; but a no inconsiderable representation of the Siphon- 

 ata (Cerithiidae, Aporrhaidoe, Strombidas, Purpuridae) is also inter- 

 spersed. The earliest fresh- water univalves belong to this period 

 (Paludina, Melania, Neritina, Planorbis). 



The corals, which are of the type of existing star-corals, may be 

 considered next in importance to the Mollusca, and, indeed, in 

 some instances, as in the Coralline Oolite, they constitute by their 

 own vast numbers the greater portion of the solid rock, not im- 

 probably the vestiges of ancient reefs. Somewhat less important, 

 but yet very abundant in certain localities, are the fragments of the 

 Crinoidea, which are most distinctively represented by the genera 

 Apiocrinus and Extracrinus, the latter having its modern analogue 

 in the Pentacrinus of the Carribean Sea; but this comparatively 

 little specialised group of the Echinodermata has, ever since the 

 close of the Carboniferous period, been on its decline, and has left 

 its place to be filled by the true urchins (Cidaris, Hemicidaris, 

 Holectypus, Echinobrissus, Clypeus, Collyrites) and brittle-stars 

 (Ophioderma, Ophiurella, Ophioglypha), both of which, but more 

 particularly the former, now for the first time acquire any special 

 importance. The star-fishes are represented, among other forms, 

 by the type of the modern Uraster. Of the articulates there is a 

 considerable development of the Crustacea crabs, lobsters, and 

 their allies and in the fine-grained rocks the remains of centipedes, 

 spiders, and true insects are not uncommon, the last comprising 

 representatives of all the recognised modern orders. To this period 

 belong the earliest Diptera, Hymenoptera, and Lepidoptera. 



