Whelks are Carnivorous and Conchs are Herbivorous. That is the main
morphological difference. Whelks as well as members of Fasciolariadae are
voracious carnivores. Conchs (members of Strombidae) are grazers. (I am
thinking of Strombus gigas, pugilus, alatus, and other shallow/ medium depth
strombidae)  I'll bet the kid was thinking of the quintessential shell
Strombus gigas. and the common Busycon Carica.  I hope this helps.
 
Sarah *stuck in the stix of Georgia for the weekend * Watson
 
ps Thank god for Laptops and local numbers!!!!!!!!!!!!