J. IMBRIE 



139 



unit known as the Fuller's Earth Rock. Although the genus 

 Ornithella displays a wide range of morphological variation at 

 each locality and a considerable overlap among geographically 

 separated populations, careful study reveals that certain modal 

 variants tend to characterize different localities, as indicated 



Ornithella 



(Q) 



Atorfh 



^ot/r/j 



Wotfon/tAyr/s 



(b) 



/♦iv/A 



■Souf/t 



Rhync/7one//o/cJs 



(C) 



Fig. 8. Diagram of some lateral changes in brachiopod communities in 

 the Jurassic Fuller's Earth Rock, (a) Changes in the outline of Ornithella, 

 simplified and somewhat exaggerated; (/;) changes in the relative abun- 

 dance and in the anterior commissure of Wattonitkyris; (c) changes in the 

 relative abundance of the rhynchonelloids (McKerrow, 1953). 



schematically in Fig. 8. Some morphological features in fact tend 

 to be symmetrically disposed about Mendips. Are we to interpret 

 these differences as the result primarily of genotypic or pheno- 

 typic variation? Even though this question must remain unan- 

 swered, the taxonomic expression of these data is in no way 

 affected. In either case the entire assemblage will be termed a 

 ( transient ) species. 



