148 GEOLOGICAL DISTEIBUTIOJT. 



preceding period. Approximately the same types are represented, 

 and while in the case of certain families a diminution in the number 

 and variety of their representatives is noticeable, in others there is 

 a corresponding increase. The corals, echinoderms, cephalopods, 

 and brachiopods have approximately the same value as in the Silu- 

 riiin period, and, indeed, several of the specific forms that are to be 

 met with in the one formation are also seen in the other. Among 

 the Devonian brachiopods we have the &st appearance of the genus 

 Terebratula, a form which has continued to flourish, although in 

 constantly diminishing numbers, from that period down to the 

 present time. With it are associated a number of other genera — 

 Strophalosia, Productus, Uncites — which likewise appear here for the 

 first time. The gasteropods are all of the holostomatous or round- 

 mouthed type — Pleurotomaria, Murchisonia, Euomphalus, Loxo- 

 nema, Holopea, Platyceras, &c. — while the lamellibranchs, which 

 show a marked increase, both in numbers and variety of form, over 

 their Silurian predecessors, belong, as far as it has been possible to 

 ascertain, exclusively to the Integropalliata, or such as are devoid 

 of a sinual inflection to the pallial line. The families represented 

 are either Heteromyaria (Aviculidffi, Mytilidse) or Dimyaria (Nucu- 

 lidse, Arcadse, Astartidse, Cardiidaa), no true Monomyarian being as 

 yet known. We meet here with the first pulmonate, Strophites, 

 a member of the modern family of snails (Helicidse), and likewise 

 with what appears to be the earliest unequivocal fresh- water inver- 

 tebrate, a mussel of the genus Anodonta, or one very closely allied 

 to it. It is here, therefore, that we have the earliest undoubted 

 traces of a fresh-water formation. 



The trilobites among Crustacea manifest a very rapid decline, 

 and, indeed, in some regions they appear to have completely died 

 out with the close of this period. The giant eurypterids — Euryp- 

 terus, Pterygotus, Slimonia — the most formidable of all known 

 living and extinct crustaceans — which first appeared in the Upper 

 Silurian formation, linger on into the succeeding Carboniferous 

 period, when they forever disappear. The Devonian deposits, as 

 has already been stated, contain the earliest remains of the highest 

 order of crustaceans, the Decapoda, or ten-footers, which comprise 

 the modern lobster and crab ; the form in question (Palaopalaemon) 

 belongs to the macrurous, or long-tailed division, and is allied to 

 the shrimps. 



