Shell small to medium sized, fusiform, elongate-fusiform or turriform, usually with high spire and well-developed siphonal canal. Suture distinct, impressed or canaliculated. Sculpture dominated by axial elements from rounded, widely set folds to dense and sharp ribs. Axials may be lacking or overridden by spiral elements on the adult whorls, but they are always well pronounced on earlier teleoconch whorls. Aperture ranging from very narrow slit-shaped to wide. Inner aperture lip usually with three or four columellar folds, subequal or (in most cases) adapical the strongest. Protoconch glossy, with no sculpture, usually multispiral, narrowly conical, or rarely paucispiral bulbous. Operculum absent (except in Ceratoxancus and Latiromitra). Eyes always present. Well-developed proboscis, valve of Leiblein, salivary glands, and (usually) accessory salivary gland. Bulky gland of Leiblein well developed or reduced, in latter case secondary tubular secretory structure connected to gland of Leiblein developed. Radula triserial with tri- or multicuspidate rachidian tooth and simple monocuspidate sickle-shaped laterals.

The family Costellariidae Macdonald, 1860 encompasses about 475 living species of carnivorous gastropods (WoRMS, 2016), and is widely distributed in tropical and temperate waters from intertidal to bathyal depths. Like many other neogastropod families, Costellariidae are traced back in the fossil record to the Upper Cretaceous, and their greatest diversity in recent faunas is recorded in the Indo-Pacific (Cernohorsky, 1970; Taylor, Morris & Taylor, 1980; Turner, 2001; Robin & Martin, 2004; Poppe, Tagaro & Salisbury, 2009). Numerous costellariid species in the genus Vexillum commonly occur sympatrically, and even syntopically, in shallow waters of the tropical Indo-Pacific, predominantly on sandy bottoms, mud, or coral rubble (Cernohorsky, 1966; Ponder, 1998; A. Fedosov, pers. observ.). A little over 30 costellariid species are also known from the Western Atlantic, and the genera Atlantilux, Nodicostellaria, and Turricostellaria are known only from American waters.
Our recent phylogenetic study of mitriform gastropods (Fedosov et al., 2015) confirmed the mono-phyly of the Costellariidae and characterized its relationships within the Neogastropoda. The ptychatractid genera Latiromitra and Ceratoxancus were recovered as a sister group to the Costellariidae; the family Volutomitridae and the turbinellid subfamilies Columbariinae and Vasinae were found to be more distantly related. Noteworthy, the family Mitridae, which was long considered most closely related, showed no affinity whatsoever to the Costellariidae.
Although admittedly new species continue to be routinely discovered and described, we have a fair knowledge of the species-level diversity of the 'mitre' families Costellariidae and Mitridae, undoubtedly as a consequence of their interest to shell collectors and amateur taxonomists. By contrast, the genus-level taxonomy in current use remains very traditional and has not significantly departed from that used by Walter Cernohorsky in his review published almost 50 years ago (Cernohorsky, 1970). A case in point is the genus Vexillum, which Cernohorsky (1966: 102) viewed as 'famous dumping grounds for an array of unrelated forms', a situation that has not significantly changed since then. With 370 valid described species (WoRMS, 2016), Vexillum currently encloses 80% of costellariid diversity. Although four subgen¬era are traditionally accepted, their boundaries remain weakly defined. Species traditionally assigned to Vexillum (Pusia) typically - though not always - possess a radula with a tricuspidate rachidian (Azuma, 1965; Cernohorsky, 1966; Fedosov & Kantor, 2010). Species classified in Vexillum (Vexillum) and Vexillum (Costellaria) both have a multi-cuspidate rachidian (Cernohorsky, 1966), and the criteria of their delimitation are rather conventional. Vexillum (Protoelongata) was recently established (Herrmann, Stossier & Salisbury, 2014) to accommodate a set of shallow-water species with a pointed multi-whorled protoconch, a black head-foot with few yellow streaks, and a tricuspidate rachidian similar to those depicted by Azuma (1965) for Pusia.
Our recent phylogenetic analysis demonstrated that the genus Vexillum is not monophyletic. In our tree two species of Zierliana were nested in the Vexillum s.s. clade, and conversely a diversity of deep-water forms fell outside this clade. The deep-water
species of Costellariidae formed four distinct clades, which, together with Thala, were referred to as 'basal Costellariidae' (Fedosov et al., 2015); these were shown to share a radula of plesiomorphic morphology with a tricuspidate rachidian, similar to the radulae found in the ptychatractid genera Ceratoxancus, Latiromitra, and Exilia.