SUMMARYBerthelinia caribbean. sp. is described from the algaCaulerpa verticillatagrowing in the mangrove swamps near Port Royal, Jamaica. The most important characters which distinguish it from related species are the position of the protoconch at two‐thirds of the shell length; the straight hinge with well developed hinge teeth; the white ligament; yellow shell rays; brown stripes on the head and mantle; the heart shaped pharyngeal bulb with a dorsal pouch; entire, denticulate radular teeth; the concentrated nervous system; and the ampulla divided into two parts. The resemblance toAnomalomya corrugataCossmann is discussed, but it is concluded that, for the present,caribbeashould remain in the genusBerthelinia.It appears to feed entirely onC. verticillatato which it remains attached by a byssus‐like thread even when the valves are closed. When irritated, it secretes a white fluid from the hypobranchial gland, but the deterrent value of this to potential predators is not known.B. caribbeamay be able to survive in brackish water, although the evidence for this is inconclus