iNCisuRA (scissurella) lytteltonensis. 39 



occasion. But I take the opportunity of dealing with an 

 interesting and suggestive essay by Osborn (10), in which 

 Gegenbauer^s admirable analysis of the different forms of 

 resemblances obtaining among animal structures is largely 

 quoted. 



In the first place Osborn makes it evident that I, in common 

 with others, have fallen into an error in using the term " con- 

 vergence " to denote the parallel stages of evolution among 

 the Fissurellidge and Scissurellidse. In the common meaning 

 of the word, convergence might appropriately be used to 

 signify that apparent approximation of structural character- 

 istics which not infrequently leads to two forms being classified 

 together in the absence of sufficiently complete information as 

 to their internal anatomy. But it has acquired a special 

 meaning, defined by Osborn as the '^ independent similar 

 development of unrelated animals, bringing them apparently 

 closer together.^' As it has been the purpose of my paper to 

 show that the families of Molluscs treated of are related, and 

 closely related, the term convergence is not applicable to 

 resemblances recurring in those families. But when I come 

 to consider whether other resemblances between various 

 mollusea should be described as due to " parallelism^' or 

 " homoplasy " I find myself in a difficulty. Parallelism is 

 defined as the " independent similar development of related 

 animals, plants, or organs" ; horaoplasy as the '' independent 

 similar development of homologous organs or regions giving 

 rise to new parts." It is added that homoplasy always 

 involves homology, while parallelism and .convergence may 

 or may not involve homology. 



In Incisura the reduction of the visceral spine, the oblitera- 

 tion of spiral sculpture, the levelling of the margins of 

 the aperture, the alteration in the shape of the foot ai-e 

 changes pai-allel to those observed in the ontogeny of a 

 Fissurellid, and they involve homogenetic organs; the 

 parallelism in this case involves homology and should be 

 called homoplasy. In Schismope the conversion of the labral 

 slit into a foramen is a change parallel to that observed in the 



