Summary.1After a preliminary investigation of the rocky shore at Kallow Point, Port St. Mary, I.O.M., an area 2 by 2½ metres was selected for special study. The whole limpet population was removed for examination, the position of each individual being marked on a map of the area.2One hundred of these were examined in detail for dimensions, colouration and radula characteristics.3With copious reference to the work of E. Fischer‐Piette it was found that two distinct types of limpet were present, corresponding toP. vulgataLinn, andP. depressaPennant. No intermediate forms were found.4Values of the fraction Radular Length/Shell Length were more or less Shell Length as found by Fischer‐Piette. Investigations as to its possible connection with age or size gave negative results.5The taxonomic significance of the radular dentition is indicated, and some interesting new types introduced. The tooth termed ‘pluricuspid’ by Fischer‐Piette is shown to possess a fourth cusp in bothP. vulgataandP. depressain addition to the three principal cusps described by this worker. The fourth cusp, conspicuous in young limpets, dwindles almost to vanishing point at maturity. A fourth cusp is mentioned by Fischer‐Piette only in the exoticP. safiana.6The remainder (c. 800) of the limpets were resolved into the two species by external examination, and a distribution map prepared, which showed howP. vulgatawas most abundant in the drier parts andP. depressain the pools and moist places.7Investigations of very young limpets showed that, while possessing rather special radular dentitions, they were nevertheless as clearly of two types a