Shell solid, subglobular with a short spire and large body-whorl. Whorls sometimes varicose. Aperture terminating in a short anterior canal, columella calloused and plicated; outer lip may be thickened and reflected, usually dentate within. The small horny operculum does not fill the aperture.
Helmet Shells are a fairly large assemblage deriving their name from an apparent likeness to the helmet of the ancients, to which the large tropic species were likened. These large forms are a well-known article of commerce as it is from them that shell cameos, etc., are carved. The smaller, more rounded species, with widened apertures, which include our Victorian representatives, are known as False Helmets.

Family Cassidae: Shell medium to large, moderately to highly inflated, thick and solid, with large last whorl and low spire, short, moderately to very strongly twisted anterior canal, and moderately coarse to very weak spiral and axial sculpture (usually dominantly of relatively weak spiral cords). Outer lip strongly thickened, varices remaining at preadult growth pauses in many genera. Periostracum very weakly developed or absent. Operculum always present. Radula similar to those of Charoniinae and Oocorythinae, but with smaller basal limbs on the lateral teeth. Jaw plates large. Proboscis relatively narrow and short, not coiled in the narrow sheath. Oesophageal gland a discrete sac, usually large and firm, black, lying well to posterior of cephalic cavity. Two large accessory salivary glands present. Stomach a relatively simple, thin-walled tube, intestine straight from stomach to anus. Two subfamilies can be recognised.

Subfamily Cassinae: Anterior canal weakly twisted, lacking prominently margined fasciole of Phaliinae; operculum narrowly oval to subrectangular, with nucleus approximately central, or near abapical end; accessory salivary glands firm and of
much-folded, broadly tubular structure, lying one behind the other so that the oesophagus is initially central but passes gradually to the right around the left (posterior) accessory salivary gland, and the oesophageal gland is at posterior right (based on Cassis [Hypocassis]fimbriata (Quoy & Gaimard) from Blanche Pt., near Adelaide, South Australia). Shells of most species are relatively elongate and heavily callused, and almost all have more prominent sculpture than those of species in subfamily Phaliinae. Morum lacks a radula (Beu, 1976a). Reynell (1905,1906) has given a good general account of the anatomy of Caleodea echinophora (Linnaeus), under the name Cassidaria rugosa (Galeodea is not preoccupied, as frequently supposed, the cited earlier name being spelled Caleodes).
Some included genera and subgenera: Cassis Scopoli, 1777, and subgenus [Hypocassis] fimbriata (Quoy & Gaimard) from Blanche Pt., near Adelaide, South. 1847; Sconsia Gray, 1847; Morum Roding, 1798, and subgenera Herculea Hanley in H. & A. Adams, 1858, and Oniscidia Morch, 1852 ( = Cancellomorum Emerson and Old, 1963, = Pulchroniscia Garrard, 1961); Caleodea Link,1807, and subgenus Galeoocorys Kuroda & Habe, 1957.

Subfamily Phaliinae nov.: shells relatively short and rounded, with weak callusing and usually with weak sculpture, almost entirely spiral; anterior canal very strongly twisted, with a prominently margined fasciole; operculum obtusely fan-shaped, with nucleus at angle between the two straight adaxial edges. Accessory salivary glands thin-walled and sacular, as in Tonnidae, Cymatiidae and Bursidae; salivary glands and accessory salivary glands lateral and clasped on either side of oesophagus well behind proboscis; oesophagus passes down central floor of cephalic cavity so that oesophageal gland is at posterior centre. Stomach a narrow, thin-walled tube with sphincter at anterior end but not at posterior, with finely and closely pleated walls; a prominent typhlosole commences obliquely at the relatively broadly open fold between cardiac and pyloric arms, and gradually reduces in height as it runs along left edge of pyloric arm. Anterior digestive gland duct beneath typhlosole on left edge at bend of stomach, posterior digestive gland duct halfway along left edge of pyloric arm (based on New Zealand specimens of Semicassis pyrum (Lamarck)).
Some included genera and subgenera: Phalium Link, 1807; Semicassis Morch, 1852 (= Xenophalium Iredale, 1927, = Xenogalea Iredale, 1927, = Tylocassis Woodring, 1928; see Beu, 1976b) and subgenus Antephalium Iredale, 1927; Echinophoria Sacco, 1890 ( = Mauicassis Fleming, 1943); Casmaria H. & A. Adams, 1853.