IBBETSON AND FORBES ON THE LOWER GREEN SAND. 193 



into existence a multitude of species which were added to those 

 which had appeared before them. This was, in fact, such a state 

 of sea-bottom as is now presented by great shell banks ; but it 

 does not seem to have lasted long, and new depositions of mud 

 appear to have extinguished some forms, whilst others suffered by 

 the change only in the diminution of their numbers. In the midst 

 of this muddy epoch, a temporary and peculiar condition of sea- 

 bottom, forming what are now called the Crackers, called forth the 

 presence of numerous mollusca, at first of various species of the 

 genus Gervillia, and afterwards of Auricula, Cerithium, Dentalium, 

 and other univalves, which appear to have enjoyed but a brief 

 existence (as species) in this locality, since similar conditions were 

 never afterwards repeated. The greater number of the Gastero- 

 podous mollusca of the English Lower Green Sand are found 

 within this very limited range. At the close of the deposition of 

 this great mass of clay there was for a time a great multiplication 

 of the individuals of certain Brachiopoda which had commenced 

 their existence in the lowest beds. Thus Terebratula Gibsi 

 suddenly appears in immense abundance, covering the bottom of 

 the sea, and predominating over the animals among which it had 

 previously been but thinly scattered. 



This lowest zone of Terebratulce marks the commencement of a 

 new state of sea-bottom where sands predominated over the clays, 

 each interval of deposition being usually marked by the presence 

 of a layer of Gryphcea sinuata, the period of rest being almost 

 always sufficient to enable the Gryphcea to attain its full growth. 

 Other bivalves are found with it, but in comparatively small num- 

 bers, and not such as are of gregarious habits. During the whole 

 of this period enormous Cephalopoda including species of Crioceras 

 and Scaphites frequented these seas, and when dead formed the 

 nuclei round which calcareous and sandy matter collected and formed 

 nodules. The death of these animals seems to have been con- 

 nected with the periodical charging of the sea with sediment ; 

 hence we find them usually alternating with the zones of Gryphcea, 

 and forming irregular bands in the intervening sedimentary 

 deposits. 



In the midst of this epoch of Gryphcea, there is a sudden re- 

 appearance of the muddy deposits, during the predominance of 

 which those animals adapted for such a sea-bottom, and which 

 had survived the cessation of the deposition of the fullers' earth, 

 again multiplied, but the species which had become extinguished 

 were not replaced by representative forms. This, however, did 

 not last long, the sand again predominating with its zones of 

 Gryphcea and lines of Crioceras nodules. 



A temporary multiplication of Terebratida sella suddenly marks 

 a change in the zoological conditions — for the Cephalopoda dis- 

 appear, although the zones of Gryphcea, which animal does not 

 appear to have been affected by the change, (probably a change in 

 the depth of the sea,) go on as before, there being, however, no 

 alternating lines of nodules. It would seem that the sea began to 



VOL. i. o 



