308 MISS MCDOXALD AXD DE. TEUEilAX OS [yoL Ixxvii, 



homceomorpliY. Examples of this are found among the gastro- 

 pods of the Zy go pleura group ; primitively smooth forms may 

 easily he confused with catas^enetic smooth forms. 



Fig. 7. — Diagram ilJusfrating transversal liomceomorplig. 



Axials stronger Spirals stronger 



than spirals. than axials. 



B' . A' 



crossing point, both equireticulate. 

 (Transversal homoeomorphs). 



A ^ B 



Axials stronger Spirals stronger 



than spirals. than axials, 



'A A'=P. Uassiciim series. BB' = P. equireticidatum series." 



(5) Size of Gastropods. 



Ahnost all the Liassie gastropods that we have studied are of 

 small size, commonly less than 10 milHmetres in length. Within 

 many series there is a steady increase in size during anagenesis. 

 The more primitiye (radical) species are as a rule small ; such 

 are ProceritJiium ogerieni and Zggopleiira transversa. If we 

 adopt the terminology recently proposed hy Mr. Buclonan,! these 

 may be called anamorphs. Certain Procerithid?e, presumably 

 Gatagenetic, generally named ExeJissa, are likewise usually small 

 and are probably catamorphs. On the other hand, some later 

 members of the Zggopleura series, described here as Z". del eta, in 

 which the ornament is catas^enetic, are biD:D*er than the normal mem- 

 hers of that group, and may be considered as megalomorphs. 



(6) Classification. 



The existing classification, as we have aheady noted, depends 

 largely on the form of the oral margin, but these characters alone 

 are insufficient to differentiate closely related stocks, since the 

 deyelopment of a canal is itself a progressive character in the 



^ ' Type Ammonites ' vol. iii (1919) p. 6. 



