302 



MISS MCDONALD AST) DE. TEI'EMAX OX [vol. Ixxvii, 



Fig. 2. — Diagram illustrat- 

 ing tlie terminology. 



(B) Measurement of Gastropod Shells. 



Apical angle is the angle made b}^ the sides of the whorls at 

 the apex. 



Spiral angle is used frequently 

 with the same meanins,', but very 

 com^monlv the angle between the 

 sides of the whorls changes during 

 growth. It seems desirable, there- 

 fore, to use this latter term for the 

 anode between the sides of the 

 whorls at anv stage. It can be 

 measured, even if only two consecu- 

 tive whorls are present. 



Sutural angle, as used in this 

 paper, is the angle made by the 

 suture and the probable axis of the 

 specimen (that is the angle ADB 

 in fig. 2). This method of measure- 

 ment is more useful in comparison 

 than the more general way of taking 

 the sutural angle to be the angle 

 between the suture and the edge of 

 the spire (that is, the ano-le ABD 

 ^P in fig. 2). ~ 



III. Gexeeal Coxsideeatio^ts. 



As we have already noted, few 

 evolutionary studies of fossil Grastro- 

 pods have been published, the chief 

 works of this kind being by Prof. 

 A. W. Grrabau and Miss Elvira 

 "Wood; these authors have shown that 

 Gastropod evolution illustrates those 

 biological j^rinciples which have been 

 demonstrated among Ammonites by 

 numerous workers. The Liassic o-astropods that we have studied 

 likewise exemplify many of these principles, and the study of 

 these features is regarded by us as being of vital importance in 

 constructing a natural classification of the Grastropods. In our 

 opinion Dr. Cossmann's classification is open to criticism, because 

 it takes little account of ontogenetic evidence, and thus may lead 

 to the grouping-together of morphic ecjuivalents on diverse lines 

 of descent.! 



Since the grouping of species that we propose in the present 

 paper is based on a study of these evolutionary changes, it may 

 prove useful to explain briefly several of the principles which are 

 of importance in tracing the inter-relationships of these Liassic 

 forms. 



^ See; for instance; E. "Wood. 29, pp. 2 <& 3. 



[The angle ADB is the sutural 

 angle ; the angle EAF is 

 the apical angle ; and the 

 angle BXC is the spiral 

 angle.] 



