DESCRIPTION
Shell size between 4 and almost 25 mm, circular or oval, usually callused. Ribs fully developed, less numerous, distant and in most species coarse. Dorsal sulcus in most cases wide and smooth, with the ribs sharply bisected and forming nodules or tubercles; spinous in some species. Spire retracted with a variable number of terminal ribs. Ventral and outer labral margin callused and usually edged. Aperture fairly wide. Labrum thickened to a variable degree. Labral teeth coarse and distant. Fossula developed. Shell dorsally beige to any shades of brown with usually three large brown patches on each side of the dorsal sulcus and many dots. Venlrum and margins more reddish or even more intense colour. Ribs usually white. The coloration in the fossil species is poorly preserved and has not been reported so far. It might have been coloured similar to the living taxa.
RADULA
Taenioglossate with the formula 2-1-1-1-2: Rachidian toolh rectangular and both margins very constricted. Base usually "M"-shaped. Outer edge variably provided with cusps though a broad cusp projects in the middle. Lateral tooth fairly complex, usually with a further prominent, pointed, inwardly directed cusp just above the main cusp. Smaller cusps follow in variable number. Marginal teeth usually simple, claw-like.
NATURAL HISTORY
The earliest members of the genus are known from the middle or late Miocene of the Atlantic Ocean and Caribbean Sea. The rich deposits of the Miocene of W Europe and the Alum Bluff Formation of Florida do not provide any species of Pusula. Only in the early Pliocene did one taxon appear in the W Mediterranean Sea. Pusula experienced greater diversity in the late Pliocene with more than a half dozen species, but the greatest diversity is seen today.

Description: The shells are usually between 10 and 20 mm in size, round, oval or oblong oval and flattened in profile. The ribs are fully developed, very strong and few in number, sometimes even wavy in structure. A smooth, broad dorsal groove separating the ribs is always present. It may also be partially filled with callosity along its length. The ribs are nodularly thickened along the groove or drawn upwards in a lamellar manner. The base is fairly flat and broadened. The lateral margins are usually sharp-edged. The aperture is narrow and often centrally located. The fossula is moderately developed. The shell coloration varies between various shades of beige or brown. In some species the dorsum is speckled or spotted. The ribs are often somewhat lighter than the background. Radula: Taenioglossus according to the formula 2 1 1 1 2: Rachidial tooth rounded, square or rectangular, sometimes considerably constricted in the middle. The lateral tooth bears further teeth on either side of the noticeably elongated toothlet. The inner marginal tooth also bears further teeth next to the claw-shaped, greatly elongated tooth. Biology: This is a thermophilic group that occurs and existed only in tropical to subtropical marine regions. They live in the littoral and at a depth of up to 100 m under stones on tunicates. Distribution: Since the Miocene of the Caribbean and the Canary Islands. In the Pliocene the distribution extended into the Mediterranean. Recently, however, only a few species are found in the Caribbean and the Pacific coast of America. Discussion: As previously mentioned, species of the genus Niveria JOUSSEAUME, 1884 have been partially classified here. However, there are significant differences in the morphology of the shells, which makes classification relatively easy. The Pliocene Pusula subpediculus d'ORBIGNY, 1852 was the only European representative of this genus, which Schilder & Schilder (1971: 18) wrongly considered a subspecies of Niveria permixta (CRISTOFORI & JAN, 1832). This error may have been due to the fact that they themselves never saw the species, which was somewhat more common in the Lower Pliocene, but drew their conclusions only from the meager illustrations in Sacco's (1894) work. The rachidial tooth of the Radula is very similar to that of the genus Triviella, which could apparently indicate a common origin. However, the species of both genera differ significantly in their biology, the appearance of the animals, and their sexual organs. Representatives of the Pusula have always been demonstrably thermophilic and were and are distributed only in tropical waters along the equator. The forms of the common phylum are generally adapted to cold water (South Africa). The species are relatively restricted in their distribution, which may indicate that, like some Cypraeidae of Australia and South Africa, they do not undergo a planktotrophic development phase. The mantle of the pusulans is relatively thin and covered with rather complex structures, whereas the mantle of the common phylum is very fleshy and usually covered only with small warts.

The species of Pusula can be arranged into four groups of species complexes. Assignment to each complex is based on the appearance of the dorsal sulcus, condition of dorsal ribs, and coloration of the shell:
1 radians-species-complex including the species alienigena, miamiensis, orientalis, radians, solandri, platyventer. The shell consists of a very wide and smooth dorsal sulcus that bisects the ribs. Occasionally the ribs form strong tubercles (solandri) around the sulcus. The dorsal coloration is rather uniform with the sulcus of lighter shades.
2 pediculus-species-complex from the NW Mediterranean, tropical to subtropical Atlantic, Caribbean, and tropical to subtropical E Pacific including the species bessei, cimex (pacei is a junior synonym), costispunctata, dadeensis, dalli, densistans, garciai, labiosa, lindajoyceae, pediculus, permagna and subpediculus. The shells have a fairly wide and smooth dorsal sulcus that bisects the dorsal ribs and the ribs forming tubercles around the sulcus. The shell colour is usually any shade of brown with dorsal patches and spots.
3 californiana-species-complex named herein as Pseudopusula gen. nov. from tropical and subtropical E Pacific and W Africa including the species californiana, canadensis, depauperata (dartevellei is a junior synonym), geigeri, parcicosta, praecursor, rota, and sanguinea. The dorsal sulcus is reduced to a depression. Ribs can be bisected at the sulcus. The shell colour is uniform with the dorsal ribs whitish.
The origin of the californiana-species-complex might be descended from P. praecursor sp. nov. of the middle Miocene of Italy. In the Pliocene the species occurred in the whole tropical to subtropical Atlantic and Caribbean. Nowadays there are two separate species groups: one is found at W Africa, and the second occurs at the NE Pacific. Future DNA research may or may not confirm that both groups have already been separated at the generic level. However, we refrain from a generic separation at this stage of knowledge.
4 vemacola-species-complex, named herein as Quasipusula gen. nov., from SE Atlantic with no dorsal sulcus.