part 4] THE EYOLUTIOT^ OF LIASSIC aASTEOPODS. 297 



14. The Evolution of Certain Liassic Gtasteopods, with special 

 reference to their Use in Steatigeaphy. By Agj^es Ieene 

 McDonald, M.Sc, & Aethije Elijah Teueman, D.Sc, 

 E.G.S. (Read May 4th, 1921.) 



[Plate XXII.] 



Contents. ^ 



Page 



I. Historical and Introductory Notes 297 



II. Terminology 300 



III. General Considerations 302 



IV. Family of the Procerithidse 309 



V. Family of the Loxonematidse 324 



VI. Other Liassic Turriciilate Gastropods 336 



VII. Geological Distribution 340 



VIII. Bibliography 342 



[For the convenience of readers, it may be premised that matter of more 

 general interest is mainly included in §§I, III, and VII, and in the earlier 

 portions of IV & V.] 



I. HiSTOEICAL and iNTEODrCTOEY NOTES. 



Gasteopods are very suitable for evolutionary studies, for the 

 shells are frequently well-preserved in fossils ; while, owing to 

 the mode of coiling, the development may be observed in any 

 complete individual without breakage of the shell. Such studies 

 on fossil Gastropods have shown that they illustrate clearly many 

 of the biological principles that have been demonstrated b}^^ inves- 

 tigators of other groups, such as the Ammonites. 



But, altliough Gastropods afford excellent material for palseonto- 

 logical researches of this nature, it is well known that apart from 

 S3^stematic Avork by numerous palaeontologists, they have been 

 comparatively neglected in this country. Further study is parti- 

 cularly desirable, since they are frequently abundant in formations 

 where the fossils that are generally used in zoning are not common, 

 and it is probable that a clearer understanding of the Gastropods 

 will make it easier to correlate such rocks. 



On the Continent a great deal more attention has been paid to 

 the classification of fossil Gastropods, by E. Koken '^ and P. Fischer,^ 

 and more recently by Dr. A. E. M. Cossmann ; the last-named author 

 has produced several valuable monographs on the various groups, 

 and his comprehensive ' Essais de Paleoconchologie Comparee ' 

 mark a great advance in the study of those fossils.-^ In the 

 course of his work that author has found it necessary to subdivide 



^ ' Ueber die Entwickelung der Gastropoden vom Cambrium bis zur Trias ' 

 Neues Jahrb. Beilage-Band vi (1888-89) pp. 305-484 & pis. x-xiv. 

 - ' Manuel de Conchyliologie, &c.' Paris, 1880-87. 

 ^ See Bibliography, 2 & 3. 



