The shells are small, rarely exceeding 10 mm. in the adult truncated form. They pos¬sess a paucispiral operculum which may or may not have a thin accessory plate of calcium on its outer surface. The shell has many whorls but most of these are lost when the animal becomes adult. Sculpture, when present, consists of rather strong axial costae which may extend from suture to suture or disappear on the whorl periphery. In a few species or members of a single species this sculpture may be entirely absent.
Members of the genus Truncalla are prosobranchiate mollusks with the sexes separate, aquatic or semiaquatic and live in the vicinity of the high water line. They rarely occur where there is much brackish water. In relationship they are close to the Bulimidae and Rissoidae.


Most species in the genus Truncatella, us is the case with nearly all true halophytic mollusks, possess a rather wide distribution. All species probably can exist for a shorter or longer period of time submerged in salt water, though they are generally found at high tide line or a little above, usually under some protective material such as seaweed, boards or rocks. Local distribution can be surprisingly discontinuous. A colony may occupy only a few feet of rubble in the midst of a long stretch of beach where living conditions are presumably identical or at least, similar. The answer is possibly that they are more or less readily transported by flotsam, become established locally for varying periods of time and then eventually die out as subsequent conditions prove inimical to their existence. So far as we can find out, little is known about the life history of members of this genus and the problem is one that needs investigation.
The peculiar habits of Truncatella have resulted in inadequate treatment of this genus by writers whose studies have been limited to either the marine or the land shells of a given region. A writer on marine shells may consider the species of Truncatella to be land forms while a writer on terrestrial species, may look upon them as belonging to the sea. Thus they may or may not receive attention, depending on the attitude of the individual author.
n the strict sense, the terms marine, land and freshwater refer only to habitat preference and are only lightly superimposed upon the much broader and more exact systematic arrangement of our mollusks. Border line families and genera and even species naturally overlap in the three major habitats so that their inclusion in any fauna! study should be based upon their ecological preference rather than their systematic position. In the case of Truncatella however, both ecological considerations and systematic relationships justify calling it a marine genus.
It is notable, however, that some primitive members of the Truncatellidae have given rise on at least two occasions to land genera: Geomelania in Jamaica and elsewhere in the West Indies and Taheitia in Polynesia and other islands in the Western Pacific.
There are several species of Truncatella that appear to have two forms: smooth or nearly smooth and strongly costate. This does not seem to be a case of sex dimorphism, as in many instances colonies of one form or the other exist as "pure" races. In other colonies these two forms are mixed, usually with many intermediate individuals difficult to assign to either form. This naturally has given rise to many names applied locally to the species or species complex in which this ambiguous sculpture occurs. Size variation is also a common character which again has complicated the taxonomy of this genus, especially in Europe. We wish in particular to focus attention on the characters of smooth and costate forms for those who have access to fresh material and those living in a region where several different colonies can be studied in the field. Such a study should prove of considerable interest and value and may solve problems of the same sort that exist on many of the temperate and tropical coasts of the world.
All known species in the genus Truncatella are mechanically truncated; that is, at or near maturity a mid whorl is plugged by a rounded septum, usually between the fourth and fifth whorls, and the unoccupied whorls are broken off. The shell walls where the septum joins the whorls appears to be slightly weakened by the absorption of the lime so that the fracture is usually fairly clean, though this is not always the case. The actual break is probably never consciously attempted by the mollusk but is generally brought about by the stresses of the environment, such as wave action or other mechanical means. This condition exists in other genera, particularly in the Urocoptidae (terrestrial) in which most genera are mechanically truncated. We cannot recall any other marine genus in which this condition exists. Under purely aquatic conditions, of course, there would be a buoyancy factor of considerable importance which would render the amputation of the unoccupied whorls less necessary. In fact, cross sections of Terebra show that many of the early whorls are completely plugged with lime. We are unable to explain why certain genera or families of mollusks produce far more whorls than they eventually occupy.
The operculum of Truncatella is paucispiral and may or may not develop a thin calcified plate on its outer surface. The amount of calcium deposited seems to vary exceedingly, even on specimens in a single locality series. Certain species from the Indo-Pacific have this character well developed; the outer surface is even ridged, the ridges curved and emanating from a common center more or less directly over the opercular nucleus.