Dr. W. B. Clarke on the Crag of Suffolk. 205 



ingly tender, that it is difficult to carry it home in a condition fit 

 for preservation. It differs from Dickieia ulvoides in its darker 

 colour, divided frond, and more tapering extremities ; besides, it 

 is an autumnal and the other a vernal species. 



Plate V. fig. 6. Frond of Dickieia pinnata, natural size. 



Note on Spirulina. 



Professor Kiitzing has described and figured eleven species of 

 this genus, but the specific differences which he relies on do not 

 seem to me satisfactory. They are chiefly the colour of the 

 stratum and comparative closeness and diameters of the spires or 

 coils. But the colours I have found to vary much, according to 

 the age of the stratum, its greater or less exposure to light, and 

 the state of the weather. In all the specimens whose growth I 

 have watched, the spires were at first very dense, but became 

 laxer after a short time ; and in a specimen of Spirulina tenuissima 

 sent me from Bristol by Mr. Thwaites, the spires were relaxed 

 at the extremities of many of the filaments, though at the middle 

 they remained compact. In Spirulina the diameters of the fila- 

 ments increase considerably as they advance towards maturity, 

 but this increase has its limits, and an acquaintance with all the 

 species is necessary to enable the observer to determine what 

 value he should assign to this character as a specific distinction. 



Plate V. fig. 5. Filament of Spirulina tenuissima, having laxer spires 

 at its extremities. 



XX. — A few Remarks upon the Crag of Suffolk. 

 By W. B. Clarke, M.D., of Ipswich. 



In the ' Philosophical Magazine ' for August 1835 Mr. Edward 

 Charlesworth published some papers upon the Crag of Suffolk and 

 Norfolk, in which he divided the formation into three successive 

 deposits. The oldest, from the abundance of zoophytes con- 

 tained within it, he termed the Coralline Crag. The second, 

 from the peculiar red or ochreous colour which pervades it, pro- 

 duced by the presence of hydrous oxide of iron, he termed the 

 Red Crag, which is characterized by the dying-out or absence of 

 a great proportion of zoophytes and the introduction of new 

 groups of testacea. The third, from its containing many fossil 

 remains of mammalian animals, he termed the Mammaliferous 

 Crag. 

 The representatives of these groups may be seen as follows : — 



Miocene Group. 

 Coralline Crag of England : Loire and Gironde in France ; con- 

 taining 17 per cent, of recent species. 



