COMPUTER-SIMULATED SHELL SIZE AND SHAPE VARIATION IN THE CARIBBEAN LAND SNAIL GENUS CERION - A TEST OF GEOMETRICAL CONSTRAINTS

A computer graphical model of gastropod shell form is used to test a h ypothesis of geometric constraint proposed to explain the disjunct dis tribution of shell forms observed in Cerion, a species-rich and geomet rically varied genus of terrestrial gastropods. The mapping of compute r-simulated forms into a morphospace of Cerion shells produces a conti nuum of sizes and shapes. Therefore, the absence of particular shell f orms is not explained by geometric constraints. Two proposed modes of shell morphogenesis at extreme ranges in size (''dwarfs'' and ''giants '') previously were thought to be exclusive routes to the construction of high-spired (''smokestack'') forms. The present study shows that t here are, in fact, multiple routes of transformation. In addition, the se routes are geometrically reversible and interconnect the members of the shell-form continuum. Thus, the possible pathways followed during the course of evolution within this genus cannot be determined until an adequate phylogenetic hypothesis has been proposed.