part 4] JURASSIC chron^ologt : lias. 281 



The specimens of WcBhnerocej^as were obtained by Dr. Eobson, 

 of Birmingham. He was informed by the quarrymen that these 

 specimens came from a temporary excavation at the base of the 

 section. On the occasion of m}^ visit this excavation was water- 

 logged, and could not be examined. There is, however, every 

 reason to believe that the statement of the quarrymen is correct. 

 The state of j)reservation of the specimens is distinctive ; they are 

 cleanly-cut casts in a bluish-gre}^ matrix with some pyrites, and no 

 fossils have been found in a similar condition and matrix in any 

 part of the quarry usually worked. 



The johnstorii Zone. — The ammonite of most frequent occur- 

 rence in this zone in the Bristol district is Caloceras intermedium 

 (Por block), but fragments attributed to C. johnstoni have been 

 obtained. 



The ijlanorhis Zone. — The true Psiloceras planorhis is rare in 

 the Bristol district ; a more evolute form, Ps. sampsoni (Port- 

 lock), and a cosfcate form, Ps. plicatum (Quenstedt), are the 

 dominant species. 



The Ostrea, tatei, and langportensis Zones. — The term Ostrea 

 Zone has generally been used to distinguish all the Lias deposits 

 below those in which ammonites occur. It is now proposed to 

 restrict this term to the beds immediately preceding the 2^lanorhis 

 zone : in these beds Ostrea liassica and its mutations attain their 

 maximum. 



Ostrece of the liassica type also occur in decreasing numbers in 

 the lower beds ; but, in these deposits, other fossils acquire greater 

 importance. 



The White Lias is characterized by Volsella (Modiola) lanc/- 

 j)ortensis, which appears to be confined to these beds. 



Between the White Lias and the maximum of Ostrea liassica 

 and its allies, there is a series of beds, more or less continuous 

 from Dorset to Yorkshire, containing in abundance JPleuromya 

 tatei. The application of this fossil to zonal purposes has, under 

 an older name {Pleiiromya croivcombeia), and in a somewhat 

 less restricted sense, already been suggested by Dr. A. Vaughan 

 in collaboration with the present writer,^ and by Dr. Rendle Short.^ 



R. Tate & J. F. Blake designate the beds in which this fossil 

 occurs in Yorkshire ' The Pleuromija Limestones.' ^ 



1 Proc. Bristol Nat. Soc. n. s. vol. x, 1903 (issued for 1901), pp. 52, 53 

 see also A. Vaughan, Q. J. G. S. vol. lix (1903) p. 400. 



2 Q. J. G. S. vol. Ix (1904) pp. 187, 188. . 



3 ' The Yorkshire Lias ' 1876, p. 38. 



