Ribbed ciliated field (1) on inner face of mantle adjacent to anus (2) and distal end of pallial oviduct (3). Head (4). Shell height 9.1 mm. Anglesey, March 2014.

Hannaford-Ellis, (1979), wrote: [In L. compressa] the CF [ciliated field] is also often heavily folded but of much smaller extent [than on L. saxatilis s.s.] and positioned near the opening of the oviduct. — The extent of the CF can, of course, vary from individual to individual and also from population to population.
The ciliated field in the above image is similar to that in Hannaford-Ellis’s figure 2, but it is not always so. Generally, the ciliated field is not an easy feature to use for differentiation of Littorina spp., and Dr D. Reid (in litt.) considers it to be unreliable. Examination of the ciliated field is not necessary for identification of L. compressa specimens as they can be easily distinguished by their shells (Hannaford-Ellis, 1979).