1854.] PRESTWICH, LONDON CLAY AND BRACKLESHAM SANDS. 445 



We have therefore, in viewing the tertiary strata in relation to the 

 underlying Chalk, to take into consideration that the existence of 

 certain classes of fossils in the former of necessity implies the non- 

 existence of other classes found in the latter deposit ; and this, even 

 should the two have been in consecutive and uninterrupted se- 

 quence in time. To estimate exactly the value of the differences ex- 

 isting between these strata, we should consequently look to those por- 

 tions of them where the terms are nearly alike. To a certain extent 

 this is practicable in the instance of the Maestricht beds and the Cal- 

 caire pisolitique, but we are not yet sufficiently well acquainted with 

 the molluscs of the latter to reason positively on the subject. "We can 

 however analyse the character of the fauna of the London Tertiaries, 

 and see how far any resemblances to forms which flourished during 

 any of the Cretaceous periods can be traced. 



The somewhat Cretaceous fades which exists, as before observed, 

 in the Lower Landenian and the Thanet Sands fossils is to be recog- 

 nised in some portion of the fauna of the London Clay itself. Thus 

 amongst the Echinodermata, the Hemiaster, a common cretaceous 

 genus, has three species in the London Clay, and but one in the 

 Barton Clay ; whilst the prevalence of Crinoids, amongst which is 

 a species of Bourgueticrinus, hitherto considered a chalk genus, and 

 three species of Pentacrinus, and the new Cainocrinus of Forbes, are 

 features more resembling those prevailing in Mesozoic than those 

 usual in tertiary strata. The two genera of Asteridce. {Astropecten 

 and Goniaster) which occur in the London Clay are common in the 

 Cretaceous strata, the Oolites, and Lias. 



The four genera of Crustacea described by Prof. M'Coy are all 

 extinct ; and one of them, the Hophparia, of which two species are 

 abundant, is a well-known form common in some of the Lower Cre- 

 taceous beds. 



The Fishes present some very marked characters, one of which is 

 the prevalence of PycnodontidcB. This extinct family, so well deve- 

 loped in the Cretaceous series, has also ten species in the London 

 Clay. The extinct genus Hypsodon is represented by two species in 

 the Chalk, and by the same number in the London Clay, above which 

 it does not range. The Notidanus is another common chalk genus 

 which is often found in the London Clay. The genus Otodus, of 

 which also several species occur in the Chalk, is a very common form 

 in the London Clay. This genus, however, as well as the last, ranges 

 higher, although both become scarcer. Altogether the eighty-three 

 species of fishes belong to twenty-three families, of which eleven 

 existed during the Cretaceous period ; several of the genera do not 

 extend upward beyond the London Clay, whilst others appear in 

 much diminished numbers. The cretaceous affinities of some of the 

 families of fishes found at Sheppey were, I find, made the subject of 



Venus {Cytherea) ovalis, figured in the ' Mineral Conchology,' one is from the Green- 

 sand of Blackdown, and another (as probahle) from the Lower Tertiary sands of the 

 neighbourhood of Faversham. It would seem also that the Pholadomya Koninckii, 

 which is now considered a tertiary species, was, I am informed by Mr. Morris, 

 originally figured with the inference of its being a cretaceous specimen. 



