386 W. IT. Hudleston — Gasteropoda of the Portland Rocks. 



as far as conld be deduced from the works of English authors. On 

 the Continent greater attention had been paid to the subject, and the 

 Portlaudian fauna of the neighbourhood of Boulogne received ample 

 illustration at the hands of De Loriol and Pellat. Up to the present 

 time, about 21 species of Gasteropoda have been enumerated by 

 those authors from the Upper Portlandian of Boulogne, and still 

 more from the very fossiliferous Lower Portlandian (zone of Am. 

 gig as). Some of these Mr. Blake has recognized in our English 

 Portland Rocks, whilst he has himself contributed other species to 

 the list ; of these latter the most important is the well-marked and 

 rather abundant Natica incisa. 



On the whole it is evident that the fauna of the Portland Lime- 

 stones of England is not so restricted as, a few years ago, people 

 were content to suppose, but it is peculiar, and it is to some of 

 these peculiarities that I would direct attention in the present com- 

 munication. 



As the concluding stage of the Jurassic system the effects of the 

 impending change would naturally be felt; the marine area was 

 becoming more restricted, and a number of creatures found them- 

 selves impounded in shallowing lagoons, and had to make the best 

 of the circumstances. To a certain extent some of the Portland Beds, 

 especially in the Vale of Wardour, may be really regarded as the 

 precursors — not only in time but in type — of the Purbecks : that 

 is to say, they were semi-estuarine, or fluvio-marine. This may serve 

 to account for the indefinite character of some of the fossils and the 

 difficulty of placing them generically. It is certain that the Port- 

 land Eocks of the Vale of Wardour, and more or less of other 

 districts, divide naturally into two very distinct types. 



The principal type is thoroughly marine. This is the one best 

 known to geologists ; it is characterized by large Ammonites, and by 

 a fair quantity of very large bivalves, such as Pecten lamellosus, Car- 

 dium dissimile, Trigonia gibhosa, etc. The species are few, and the 

 specimens too often occur as internal moulds only ; still the general 

 facies is very recognizable, and is the one which is to be seen in the 

 majority of quarries, where people have been content to pick up a 

 few of these big ugly fossils, and to come away with the notion that 

 little else was to be obtained. It should be observed that some very 

 interesting fossils have been found of late even in this type of rock. 



Far otherwise is the facies of the second type, which may possibly 

 have been connected with the discharge of some river. It is dis- 

 tinguished by one or more species of Cerithiwn in great abundance, 

 together with Neritoma sinuosa and Cyrena rugosa in greater or less 

 quantity. Sometimes all three are present ; occasionally one or the 

 other may be rare or absent. With these are associated some of the 

 shells of the first or megalomorphic type — but always much smaller — 

 together with many species not found in the first type. It is very 

 interesting ground for the palasontologist, in those beds where the 

 shells have been preserved, and it is in this direction that we may 

 hope in future to enlarge the list of Portland fossils. 



All the indications 2:0 to show that considerable difference of con- 



