iNcisufiA (scissurella) lytteltonensis. 'SS 



elongate and scalariform. Such an ancestral form would not 

 be far removed from a Pleurotomariaj but would differ from 

 it in the development of a double columellar muscle and in 

 the tendency to acquire a secondary symmetry always 

 correlated with the doubling of this muscle. The Scissurellidse 

 have retained most of the features of this parent form, but 

 have undergone considerable specialisation iu the nervous 

 system. The Fissurellid branch must early have acquired a 

 " sessile " habit, and have been much modified in connection 

 with it, but its members have lai-gely retained the primitive 

 condition of the pedal centres. The Scissurellidge, though for 

 the most part constant to the primitive type, are also under- 

 going modification in the same direction as the Fissurellidae. 

 In Incisura the visceral spire is reduced, the shell is becom- 

 ing thick and solid, the spiral sculpture is absent, the margins 

 of the aperture are in one plane, the foot is becoming short 

 and broad, and its whole organisation is indicative of a semi- 

 sessile habit. Further specialisation along these lines would 

 give it Fissurelliform or rather Emarginuliform characters. 

 It is interesting to note that another member of the family, 

 Schismope, while retaining- its spiral coil and widely open 

 umbilicus, has undergone specialisation in another direction, 

 for the labral slit has been converted into a foramen by the 

 approximation of its edges, so that although distant from the 

 margin it is connected with it by a suture. In this respect 

 it closely resembles Semperia, a sub-genus of Emarginula. 

 Semperia leads on to Rimula, and as we have seen there are 

 Emarginuliform and Rimuliform stages in the development of 

 Fissurella. This is an undoubted example of the develop- 

 mental stages of one form resembling the adult stages of 

 other forms, a phenomenon the occurrence of which some 

 persons are inclined to deny nowadays, though the evidence 

 in favour of it is very large. 



The pai-allel stages of evolution among the Scissurellidse 

 and Fissurellidee afford interesting examples of the pheno- 

 menon of convergence, and illustrate a principle which, I think, 

 has not been sufficiently attended to in drawing inferences as to 



