part 4] THE EVOLUTION OF LIASSIC GASTEOPODS. 341 



and Upper Lias. In each of these groups evolution progresses in 

 a similar direction : the earher forms appear to be rather sparsely 

 costate, while generally the later forms are more iinely ribbed, and 

 the ribs are more distincth^ curved (fig. 14, p. 327). These 

 changes take place somewhat rapidly, and species become distinct 

 by the curve of the ribs and by the degree of prominence and the 

 position of spiral lines. On the whole, it is more difficult to 

 identify species among the Loxonematida) than among the Pro- 

 cerithidse, since fewer characters are available. 



It will appear from the facts set forth above that the evolution 

 of these gastropods generally proceeded much more slowly than 

 the evolution of many of the Ammonite families, and therefore 

 correlation by gastropods will be much more difficult than correla- 

 tion by ammonites, it must be remembered, however, that some 

 series of the Procerithidse evolved fairly rapidly, and that some 

 species of such series have been found to characterize definite 

 horizons ; it is extremely probable that, if care be taken in identi- 

 fying the species, such forms would prove extremely useful in 

 correlation. 



On the other hand, there will be considerable difficulty in 

 identifying many of these specialized gastropods ; as a result of 

 evolution along parallel or nearly parallel lines homoeomorphs are 

 of frequent occurrence, and, since these have not always been 

 developed simultaneously, careful discrimination and the use of 

 ontogenetic evidence are necessary to distinguish species which 

 may be superficially alike, but may be widely separated in time 

 and affinity. 



Further, the long vertical range of these groups of Oastropoda 

 makes their use in stratigraphical work extremely difficult, because 

 it is impossible in many cases, even approximately, to date a 

 specimen without accurate naming, while it is, of course, possible 

 roughly to determine a zone by means of an ammonite without 

 expert knowledge or careful examination. Indeed, it seems that 

 the Gastropod families that we have been discussing, the Pro- 

 cerithidse and Loxonematidse, are to be compared rather with such 

 slowly-evolving Ammonite families as the Lytoceratidse and Phyl- 

 loceratidse, than with the Amaltheidie or the Arietida^. 



In conclusion, we wish to thank those who have helped by 

 lending or giving specimens, or allowing access to collections in 

 their charge, namely : Mr. J. Gr. Hamling, Dr. F. L. Kitchin, 

 Dr. W. D. Lang, Mr. L. Kichardson, Prof. H. H. Swinnerton, and 

 Mr. J. W. Tutcher. We are also indebted to Mr. S. S. Buckraan 

 and Dr. A. E. M. Cossmann for several suggestions. One of us 

 (A.E.T.) wishes to acknowledge a grant from the Royal Society 

 Committee which has enabled him to collect extensively in 

 Somerset. 



