part 4] THE EYOLUTION OF LIASSIC GASTROPODS. 305 



(2) Acceleration. 



In almost every series o£ related gastropods that we have studied 

 the later members are more highly accelerated than the earlier. 

 This is illustrated in a series of forms, probably genetically con- 

 nected, which includes Procerithmm slatteri. Three members 

 of such a series are shown in fig. 4 (p. 304). It will be noticed 

 that : 



(1) The two-spiralled stage is retained later in the less-accelerated forms 

 (%. 4 a, iv). 



(2) The axials appear earlier in Procerithium cf. slatteri than in the less- 

 accelerated species (compare fig". 4 c iv with 4 a v). 



(3) The network with four spirals is attained earlier in Procerithium cf. 

 slatteri (fig. 4 c vi). 



Numerous series of Liassic Loxonematidse show a comparable 

 acceleration, the earlier members having several smooth whorls 

 near the apex, while the more highly accelerated forms have only 

 one or two. Owing to slightly different rates of acceleration, it 

 it is not uncommon to find that specimens collected at the same 

 horizon show some variation. 



(3) Catagenetic Gastropods. 



In a few catagenetic species the ornament is retarded, when 

 compared with the development of ornamentation of those species 

 from which they are presumably derived. One of the best in- 

 stances of this is Procerithium. numismcde^ the development of 

 which is compared with that of P. ogerieni in fig. 5 (p. 306). It 

 will be noticed that : 



(1) P. nmnismale retains the two-spiralled stage longer than P. ogerieni. 



(2) Axials are not developed in P. numismale until half a whorl later than 

 in P. ogerieni. 



(3) The reticulate pattern is never so pronounced in P. mimismale as in 

 P. ogerieni. 



In the later stages it will be noticed that the axial ornament 

 becomes feeble, the middle spiral losing its tubercles before the 

 two outer spirals, and the last whorl suggesting a return to the 

 two-spiralled stage. In quite a number of the^ S23ecimens in 

 which the ornament shows these retrogressive characters there 

 is also a tendency to produce a pupoid form of shell, particularly 

 among the Procerithidse. These forms have generally been called 

 Exelissa ; but it is here suggested that they are catagenetic 

 descendants of various species of Procerithium (see p. 317). 



(4) Homoeomorphy. 



The chief difficulty in the natural classification of Liassic gas- 

 tropods (indeed of most fossils) lies in the recognition of morphic 

 equivalents or homoeomorphs. Species occupying comparable 



