4 FAUNA OF THE CORNBRASH. 



only those forms represented by specimens obtained from above the base line 

 assumed either by myself or where there is no confusion possible. 



This line is recognised as being the basal bed which shows two or more of 

 the following characters : It is rubbly, nodular, or without regular minor strati- 

 fication ; it is an " aggregate " crowded with fossils, many of which may be 

 heterochthonous ; it is often ferruginous or phosphatic. Such are recognised as 

 basal beds, often unconformable on a large scale, as in the cases of the Cambridge 

 Greensand, the roof-beds of the Brora Coal, or the nodular beds at the base of the 

 Speeton Clay. Similar beds are found associated with Mac. macro cejphalus or Clyd. 

 discus at various localities on the Continent and in England, and the base of the 

 rubble beds with Avicula echinata is taken in any case of doubt as the base of the 

 Cornbrash. 



The exact definition of the upper limit is not of so great importance, for the 

 change of the matrix from limestone to clay will of itself cause some change in the 

 fauna; but, as H. B. Woodward observes, "there is no palreontological break in 

 the South of England or elsewhere between the Cornbrash and the Oxfordian 

 series, for in the Kelloway Rock we find more or less abundantly some of the 

 characteristic fossils of the Cornbrash." Mac. macrocephalus has, however, been 

 recorded to ascend to the Kelloway Rock in Wiltshire, but the evidence is not 

 satisfactory to me, for in many j)laces where the lowest Oxford Clay is exposed 

 the first Ammonites to occur are always of the varieties Icoeuigi, gowerianus and 

 modiolaris. 



Critical Examination of the Sections which have yielded Cornbrash Fossils. 



The following is a description of the localities which have yielded Cornbrash 

 fossils, together with critical reasons for the rejection of others which have been 

 supposed to yield them. It will be seen that in most of the former cases the basal 

 rubble beds are exhibited, below which no fossils are recognised as belonging to 

 the Cornbrash. 



1. Radipole, Weymouth backwater. — This is the well-known locality where 

 the end of the backwater faces the town, exposing the whole sequence, from the 

 Oxfordian downwards. The highest bed on the east of the section is only exposed 

 on the foreshore, beneath the supports of the drain-pipes ; it is characterised by 

 species of Goniomya, and Pholadomya (Bed a). On the other side of the 

 bounding wall the following series can be made out : 



Fb. in. 



1. Solid blue limestone . . . . . . .10 



2. Soft and brashy, with irregular thin doggers . . . . 10 



3. Lenticular compact white limestone . . . . .18 



