part 4] THE ETOLIJTIOlSr OF LIASSIC aASTEOPODS. 



307 



young, has its spirals stronger than its axials, and the ornamen- 

 tation is not normally reticulate until rather late in ontogeny. In 

 P. liassicum, on the other hand, the early ornament consists 

 of strong axials and feebler spirals, and in development the axials 

 become relatively w^eaker, giving rise to a reticulate ornament 

 similar to that of P. eq^uireticulatttm. 



Fig. 6. — Diagram illustrating the development of tivo homoeo- 

 morphic gastropods of different descent (transversal 

 Tiomoeomorplis) . 



Vll 



V. 



VIII. 



a, = Procerithiiim eqtiireticulatum, sp. nov., Lower Lias (armatus zone), 

 railway-cutting-, Toddington (Gloucestershire). L. Eichardson coll. 

 L.G. 7. 



b = Procerithiiim liassictim (Tate), Middle Lias, Eston (Yorkshire). Tate Coll., 

 Museum of Practical Geology, No. 7968. 



In each of these series, therefore, a stage will be reached in 

 which the ornamentation will consist of spiral and axial elements 

 of equal strength forming a fairly regular network. The later 

 species of each series may pass beyond this stage ; but their sepa- 

 ration would not be difficult if the axials continue to change in 

 strength. Yet at the ' crossing-point ' of the two lineages, the 

 members of the two series have at least a superficial similarity, 

 especially if they possess similarly-shaped whorls ; and their rela- 

 tionships are only made clear by a study of their ontogeny. 



One other type of homoeomorphy has also been indicated by 

 Mr. S. S. Buckman in his work on Ammonites,^ namely, cyclical 



^ Op. jam cit. p. 166. 



