260 THE CARBONIFEROUS SYSTEM. 



locomotion at the surface of the water. In addition to these shells, we 

 have several species of univalves, resembling the modern Naticas, or 

 whelks and periwinkles, and a number of bivalves belonging to the 

 class Lamellibranchiata, and to the genera Avicidopecten, Macroden, 

 Cardiomorpha, etc., all of which may be considered as representatives 

 of the modern bivalve shell-fishes like the Scallops, Mussels, Cockles, 

 etc., though of distinct species. 



Other bivalve shells are very numerous, especially a species of 

 Terebratula, two otSpirifer, an Athyris, several species of Rhynchonella, 

 and two of Productus. These shells belong to a tribe (the Brachiopoda) 

 differing in some important particulars from the ordinary bivalve shell- 

 fish, and remarkable as having been very numerous in ancient periods 

 of the earth's history, and comparatively few now. Some of the most 

 abundant species of these genera are figured in subsequent pages. 

 The Terebratula is not unlike some of the modern representatives 

 of the family. The Rhynchonellas are still represented in our 

 modern seas by the Parrot-bill Rhynchonella [R. psittacea), now 

 found, though rarely, on the coasts of Nova Scotia. The Productus 

 is remarkable for the great convexity and comparative magnitude of 

 one of its valves, which, as has been conjectured by an eminent zoolo- 

 gist, may have been the lower valve, and have formed a sort of cup 

 containing the animal, and closed by the smaller valve. The Spirifer 

 and Athyris are distinguished by the presence within the shell of two 

 spiral stony threads, twisted like cork-screws, and connected with the 

 support of the long spiral arms with which all these creatures were 

 provided. These screws are often finely preserved in the Windsor 

 limestone. I may mention here, that in all the Carboniferous lime- 

 stones of Nova Scotia the shells of this family are usually found with 

 the valves closed and the interior often hollow. This shows that they 

 were not dashed about by violent waves, nor exposed to be filled with 

 fine mud. Yet it does not prove that the death of the animals was 

 sudden, for the hinge of the modem Rhynchonella and Terebratula is so 

 constructed that it does not gape when dead, like other bivalve shells ; 

 but when dead and empty, the hole or notch in the hinge for the pe- 

 duncle, by which these shells were attached, would admit mud, had this 

 been present, which in many instances seems not to have been the case. 

 The appearances are those which should occur in a bed of shells 

 gradually accumulated in deep and clear water. 



Descending a little lower in the animal scale, we have fragments of 

 the stems of Crinoids, which were complicated starfishes, mounted on 

 a stalk. A pretty little branching coral is also very abundant, and 

 with shells, which are entangled in great numbers among its branches, 



