part 4<'] THE EVOLUTION or LIASSIC GASTEOPODS. 811 



From these forms, which may perhaps be regarded as constituting 

 a central stock in Sinemm-ian times, many other forms evolved. 



(i) Some by the addition of secondary spirals, and increase in the number 



of axials : for example, P. liapsicum. 

 (ii) Some species become carinate by the accentuation of one spiral, 

 (iii) By the loss of axial ornament in catagenesis certain forms become 



ornamented with spirals only, 

 (iv) By gradual loss of ornament and contraction of spiral angle, forms 



become pupoid or of Exelissa type. 



These changes must not be considered by themselves as indi- 

 cating the natural grouping of the various species, but simply as 

 showing the main trend of evolution ; there is no doubt that each 

 of these changes has taken place in several different stocks, some 

 of these contemporaneously, others at different times. 



Comparison of Procerithiuin with modern Cerithiidse. 



In the paper already cited, ^ Miss Elvira Wood has described in 

 some detail the development of several i-ecent and Tertiary Ceri- 

 thiidse. Among these are some differences due to relative accelera- 

 tion ; but in many forms the embryonic whorls and protoconch 

 are about one and a half whorls in length, after which one or two 

 spirals appear ; if onl v one spiral is developed first, a second 

 is added at a very early stage above the first. Axial ribs are 

 developed at a somew^hat later stage, often as early as the third 

 whorl, and before the appearance of the third spiral ; secondary 

 spirals are intercalated at later stages. 



It is interesting to compare the development of early Liassic 

 species with these recent Cerithiidse. In each case the order 

 of development of the various features is similar ; but the Tertiary 

 and recent species are, on the whole, more accelerated than Liassic 

 forms. For instance, the axial ornamentation in Tertiary forms 

 usually appears during the third Avhorl, while in Liassic forms it 

 rarely appears before the fifth whorl ; the addition of secondary 

 spirals in recent forms proceeds more rapidly than in Liassic 

 species, and while the adult whorls of early Liassic species have 

 typically three spirals and rarely more than four spirals (and 

 Inferior Oolite species have typically four 2), the recent species 

 often have ten or more spirals. 



Notwithstanding these differences due to the relative accelera- 

 tion of the later forms, there is still a remarkable similarity 

 in the development of these two groups, which suggests close 

 relationship. 



Procerithium oaERiENi (Dumortier). (PI. XXII, fig. 8 & 

 text-fig. 5 a, p. 306.) 



CeritMum ogerieni Dumortier (5, pt. 2, 1867, pi. xlv, fig. 6). 

 Procerithhim ogerieni (Dumortier) Cossmann (3, text-fig. 15, p. 45). 

 Cf. CeritMum deeoratum Moore (11, pi. xvi, fig. 14). 



1 29. 2 9^ pp^ -[45^ 146^ 



