1850.] PRESTWICH ON THE LOWER TERTIARY STRATA. 279 



After deducting the Woolwich and Heme Bay species from the 

 fauna of this period, there remains nineteen out of thirty-one described 

 species which are not found in the underlying deposits. They form 

 a distinct and well-marked group, and the individuals of the species 

 are often extremely numerous. The principal species are the Pa- 

 nopcea intermedia, Cytherea obliqua, C. ovalis, y&y J., Cai^diumnitenSj 

 C. Plumsteadiense, Pectunculus brevirostris^, Natica glaucinoideSy 

 N. Hantoniensis, and Rostellaria Sowerbyi. In the western districts 

 the Bitrupa plana is also particularly abundant, and the Pyrula 

 tricostata is common, but both these die out or become very scarce 

 as they range eastward. 



Between the eastern and western districts there is also a space (in- 

 cluding Woolwich, Upnor, Boughton) where, owing probably to the 

 more brackish state of the sea, as evidenced by the great and sudden 

 abundance of Melania and the several species of Cyrena, many other 

 marine genera disappear. A few, as the Panopcea, Rostellaria, one 

 species of Cardium, and Calyptrcea, are however persistent through- 

 out (see Table A.). Notwithstanding this interruption, the fauna at 

 Heme Bay presents a remarkable similarity to that which flourished 

 at the same period at Hedgerley, Reading, and Clarendon Hill, modi- 

 fied only by the introduction of the few shells before mentioned. 



It is to be observed, however, that the fossils of this deposit, al- 

 though they have so persistent a range from west to east, decrease 

 rapidly to the north-east, and from their nearly total absence in 

 Essex and Suffolk, an argument might be brought against the identity 

 which I have there given to that deposit in Sections Nos. 1 7 to 20. On 

 structural and lithological grounds I have before argued in favour of 

 this identity, and I cannot view this absence of fossils as a militating 

 argument of much weight against such a supposition. If it occurred 

 on a line where the same bed in adjacent sections was very fos- 

 siliferous, then the question would be attended with some difficulty ; 

 but in this instance we are led almost naturally to anticipate their 

 disappearance, from their rapid decrease as we proceed eastward on 

 their northern line of outcrop from Hedgerley, where they abound, 

 by Watford, where they are far less numerous, to Hatfield and Hert- 

 ford, where they are comparatively scarce. Their decrease in this 

 direction is in that ratio, that their rarity or absence in Essex and 

 Suffolk presents no anomaly. The only fossil constantly present is the 

 tooth of a species of Lamna, probably the Lamna elegans of Agassiz. 



In order better to show the range of the organic remains of this bed, 

 I have added the accompanying Table A. with a list of all the de- 

 scribed species and of two or three of the more important undescribed 

 ones. As a group, it will be observed that these species are essentially 

 those which we find afterwards characterizing the London clay, and 

 that the other species, which range from the beds below, are fewer in 

 numbers, and possess usually a very wide vertical, although at this 

 level they exhibit a very limited horizontal, rangef . 



* I do not give the Pectunculus Plumsteadiensis, as it seems to me to be doubt- 

 ful whether this species is not a variety of the P. brevirostris. 



t The breaks in the range of many of the species will probably decrease as the 

 examination of this bed is made more complete at some of the localities. 



