264 Br. A. E. Trueman — 



fifty, and preferably more than a hundred, adult specimens, from 

 a particular horizon. It will generally be sufficient to note the 

 coiling and any other prominent characters, and it is not usually 

 necessary to preserve large numbers of specimens. There is 

 frequently some difficulty in judging exactly the number of whorls 

 present in uncleaned specimens, particularly in the more advanced 

 members of the series. If all the records are made by the same 

 observer, however, any small inaccuracies do not appear to be of 

 very great practical importance. From these data a graph may be 

 constraoted and the zone may be determined by comparison with 

 graphs made from known horizons. . 



Eelation of G. incurva to other Gryphjeate Series. 



It is not necessary in this paper to describe other gryphseate 

 forms, but it may perhaps be usefully noted that other Gryphseas, 

 such as G. cymhimn and G. maccullochi, have probably been evolved 

 from oysters. Indeed, it is extremely likely that these gryphaeiform 

 shells have been evolved repeatedly during the Jurassic and 



Fig. 6. — A large specimen of GryphcBa cf. incurva from the Agassiceras sub- 

 zone, Kilmersdon Colliery Quarry, Eaclstock. 



Cretaceous from species of Ostrcea that are similar and are presumably 

 closely related. In other words, " Gri/phcea " is a polyphyletic 

 group, containing species evolved along many different lines.^ 

 Therefore, the name Grjjphcea can only be applied strictly to one 

 of these series, and each such series should receive a separate generic 

 name ; but until more of their characteristics are known, at least, 

 it appears undesirable to add to the existing confusion by creating 

 new names for each group. Indeed, as homoeomorphs in some of the 

 series are almost or quite identical, it would probably be impossible 

 to distinguish many of the genera even though names were available. 

 Some of these gryphseate series may be distinguished by the 

 measurement of the angle of the spiral (see Fig. 3) and by the form 

 of the beak. 



^ The parallel evolution of these numerous series, each passing almost 

 inevitably through comparable stages, and following what may be regarded 

 as a pre-determined " programme ", affords an example of what Dr. F. L. 

 Kitchin has called " programme-evc^lution ". (See W. T). Lang, Catalogue 

 Cret. Bryozoa, vol. iii, 192], p. xviii.) 



