320 RELATIVE AGES OF THE CRAG 



and in the coralline crag only 19 per cent., of recent species 

 of true Mollusca. 



That the crag of Norwich was newer than that of the red 

 crag of Suffolk, had been already implied by Mr. Charlesworth 

 when he suggested that shells of the former had probably been 

 washed into the Norwich beds ; and both he and Mr. Wood 

 had recognized in the assemblage of Norfolk shells, a nearer 

 approach to the existing British Fauna ; but it is most satis- 

 factory to have these conjectures borne out by a detailed ex- 

 amination of the Norfolk and Suffolk shells, a task, in the 

 execution of which Mr. Wood and myself have had through- 

 out the assistance of Mr. George Sowerby, without whose 

 experience and knowledge of the living shells, we could not 

 have arrived at such positive conclusions. 



Only two species of freshwater shells have been hitherto 

 found in the red crag of Suffolk, and these Mr. Wood col- 

 lected at Sutton, namely, three individuals of Auricula myo- 

 sotis, and a single specimen of Planorbis marginatus, belong- 

 ing to that variety in which the keel is slightly prominent. — 

 This same variety of Planorbis, as well as the Auricula, have 

 both been discovered in the Norwich crag. Amongst the other 

 freshwater species in this crag, I may mention Cyrena trigo- 

 nula, which occurs both at South wold, and at Crostwick near 

 Norwich. The land shells consist of Helix hispida and 

 H. plebeium, common British shells, and two perfect speci- 

 mens of a Helix found by Capt. Alexander at Southwold, 

 which bears a very near resemblance to the Helix turonensis, 

 so common in the faluns of Touraine. Of the 92 marine shells 

 all, with two or three exceptions, are either species found in 

 the red crag, or now living, so that a very small number seem 

 to have been peculiar to this period. 



The most difficult point to determine in respect to the fos- 

 sils of the Norwich crag, is the propriety of excluding certain 

 species on the ground of their having been probably washed in 

 from an older bed. The mere circumstance of shells being com- 

 mon to the red crag, and, at the same time, of extinct species, 

 raises in itself no fair presumption against their belonging to 

 the period of the Norwich beds. For some of the commonest 

 shells, such as Mya lata, Tellina obliqua, Astarte plana, 

 Tellina pretenuis, Nucula Cobboldics, Auricula pyramidalis, 

 and some others, are extinct species, and found also in the 

 red crag of Suffolk : yet no one can doubt that these lived 

 in the sea of the Norwich crag, as they abound in it in a good 

 state of preservation, although some of them are fragile shells, 

 and the Acephala have occasionally both valves united. Nor 



