Virtually all surveys of shell disease in monitoring studies have relied entirely upon gross evaluation (Sandifer & Eldridge 1974; Sindermann 1989; Ziskowski et al. 1996). Our studies suggest that while gross examination might be used as a crude indicator of shell disease prevalence, it cannot be used to reliably indicate histologic lesion severity. The latter is important because lesion severity probably relates to the potential harm that such damage could inflict on the host.