﻿CARBONIFEROUS PERIOD 



55 



UNIVALVED 

 MOLLUSCS 



and when it came, the pent-up energy expressed itself not 

 only in vast numbers, but in some colossal forms. The 

 champion of the Period dwelt in a shell measuring nearly a 

 foot across (P. giganteus). Brachiopods in butterfly-shells 

 were also numerous, and flourished on European, North 

 American, and Asiatic coasts (Spirifer). 



Lampshell-life, therefore, although long past its prime, and 

 robbed of much of its old variety, was by no means going to 

 pieces. 



No notable developments appear to have taken place polyzoans 

 among polyzoans. Open-mouthed colonies were still in 

 force on the sea-floor at various depths ; but they had 

 been surpassed in number by the later-developed com- 

 munities in which the zooids lived in cups partially protected 

 at the orifice. 



Marine snail-life exhibited some changes : but many old 

 genera, dating as far back as the Cambrian, still held on 

 their way. A few new forms were now to be seen in shells 

 with loosely coiled tubes, rising in spiral fashion (Vertnetus). 

 This development of the edifice, though peculiar, was not 

 unsuccessful, for some marine snails — of extremely passive 

 character — are still in tenements of this design. 



Early types of scallops, wing-shells, ark-shells, and mussels, bivalved 

 and various double-shelled molluscs in intermediate con- molluscs 

 ditions of development were more fully and widely repre- 

 sented. The emigrants to brackish and fresh waters were 

 also on the increase. 



Among the creatures combining pearl-oyster and mussel 

 affinities some very large forms were in evidence (Aviculo- 

 pinna) ; and oysters were appearing with which the edible 

 oyster of our own time retains more or less close relationship 

 (0. nobillisima). 



Among new forms were animals of the date-shell type 

 (Lithophagus) ; and primitive file-shells were also to be 

 seen (Lima). Other forms were appearing with the edges of 

 the mantle modified into tubes or siphons for conducting 

 water to the gills (Allorisma). These siphons could be with- 

 drawn completely within the shell-valves, and were probably 

 developed in connection with burrowing habits. 



