312 Proceedings of the British Association for 1850. 



and only twelve species of moUusca and Crustacea had been 

 determined, — whereas more than seventy were now enumer- 

 ated. The strata examined occur along the coast between 

 Weymouth and Dorchester, at Durlestone Bay, near Swan- 

 age, and in the quarries at Swindon, Wilts, where the bar of 

 the Purbeck series is exposed, and corresponds exactly with 

 the Dorsetshire beds. After describing these strata. Prof. 

 Forbes said : — It is very remarkable that, whilst the Purbeck 

 can be divided into upper, middle, and low^er, each with its 

 peculiar assemblage of organic remains, the lines of demarca- 

 tion between them are not lines of disturbance, or physical 

 or mineral change. The features which attract the eye, such 

 as the dirt-beds, the dislocated strata at Lulworth, and the 

 cinder-bed, do not indicate any breaks in the distribution of 

 organized beings. The causes which led to a complete change 

 of life three times during the deposition of these freshwater and 

 brackish strata must be sought for, not simply in a rapid or 

 sudden change of their area into land or sea, but in the great 

 lapse of time which intervened between their epochs of depo- 

 sition. A most striking feature of the mollusc Fauna of 

 the Purbecks is this, so similar are the generic types to those 

 of tertiary freshwater strata and those now existing, that 

 had we only such fossils before us, and no evidence of the 

 position of the rocks in which they are found, we should be 

 wholly unable to assign them a definite geological epoch. — 

 A comparison of these fossils with the collections from the 

 Hastings sand and Weald clay leads the author to believe 

 that the Fauna of the middle and upper Weald en series 

 is almost entirely distinct, as far as species are concerned, 

 from those of the lower or Purbeck division. Some of the 

 species reputed identical prove to be distinct ; and others are 

 derived from certain anomalous beds near Tonbridge WeHs, 

 believed to be true Purbeck strata by the author. The ex- 

 cellent monograph on the Wealden of N. Germany by Dunker 

 and V. Mayer, affords the strongest confirmation of these 

 views, showing that the Fauna of the German Wealden essen- 

 tially corresponds with the British, and that the organic con- 

 tents of the Purbecks of the Continent correspond with ours 

 and differ almost entirely from those of the upper beds. 



