European marine Mollusca: notes on less well-known species. VIII. Caecum armoricum De Folin, 1869

A number of years ago, in a revision of the European Caecidae, the first author considered the species Caecum armoricum De Folin, 1869, to be an aberrant form of the widely distributed Caecum glabrum (Montagu, 1803) (Van Aartsen, 1977: 8). This opinion was based on the type-lot in the De Folin-collection in the Museum National d’Histoire Naturelle in Paris, from Mesquer (France), and one other specimen in the Jeffreys collection in the National Museum of Natural History in Washington (no. 189852) which was obtained by Jeffreys from De Folin. This last specimen originated from St. Malo and is rather strongly curved, thus indicating that it is a young specimen (second growth stage). The specimen is glued onto a glass-slide, thus making accurate study of the septum impossible. Last year the second author collected quite a number of specimens (about 100) of a species of Caecum at Penthièvre (Southern Brittany, France) which were quite clearly not identical with Caecum glabrum nor with Caecum trachea (Montagu, 1803). Restudy of the De Folin collection showed that, apart from the type-lot, there existed a second lot of five specimens originating from “St. Malo, Embouchure de la Rance”. This second lot is also mentioned by Kisch (1959: 25). These specimens are also glued onto glass in the characteristic blue slide used by De Folin. Although these specimens cannot be used as type material, because they are not mentioned in the original description by De Folin (1869: 148, 149), it was apparent that the specimens from Penthièvre are identical with those from St. Malo. The specimens of St. Malo, however, are somewhat younger and therefore the septum is not yet fully developed as it is in some of the specimens of Penthievre. The fully developed septum clearly shows an auriculiform appendage on the septum somewhat reminiscent of that of Caecum auriculatum De Folin, 1868, but differently placed. Because of the relative rarity of this species, we thought it interesting to publish some drawings of the full-grown Caecum armoricum and its septum in figs. 1 and 2, as well as of the second growth stage in fig. 3.