Pectinodontinae: The shell of pectinodontines typically has prominent radial and concentric sculpture which gives it a cancellate appearance. This form of sculpturing is uncommon in other neat-shore patellogastropod taxa. Shells with only concentric growth lines do occur: known fossil species are also much less cancellate than living taxa. Apex position varies from just slightly anterior of centre to the anterior quarter of the shell. Most specimens also show a characteristic early growth phase in which apertural surface area expands rapidly with little increase in shell height. After this initial phase of rapid dilation subsequent shell growth is more equally divided between apertural increase and increase in shell height. It is this ontogenctic change that produces the dimpled apex characteristic of pectinodontinc species. The pectinodontic shell is composed of four shell microstructure units — the exterior layer is homogeneous and followed by a narrow foliated layer, a commarginal crossed-lamellar layer, the myostracum. and a complex crossed-lamellar layer. The radular tooth morphology is unique among patellogastropods. The inner lateral tooth is unicuspidate, the second lateral is uni- or bicuspidate, and the exterior outer lateral tooth is multicuspidate with between seven and twelve cusps per tooth. All three teeth are in close juxtaposition forming a narrow, inverted V-shaped, row alignment. [The Southern Synthesis]

The Pectinodontidae, including the vent taxon Bathyacmaea, are the sister group of Lepetidae. Pectinodontid species are found world-wide, at bathymetric ranges from about 300—6000 m. The highest species diversity is found in the equatorial region, with fewer species occurring in temperate regions (Lindberg, 1998b). Similarly, lepetids are strictly subtidal, but are restricted to cold waters. They can occur just below the low water mark in the Northern Hemisphere, but extend to depths in excess of 5000 m towards the equator (Moskalev, 1977). The origin of these two families adapted to anoxic condition is estimated at 98-216 Ma, and the divergence between Bathyacmaea and Pectinodonta at 31—98 Ma. The hvdrothermal vent habitat has attracted much interest, including questions about the origin of its fauna. Recent molecular studies of several ecologically dominant vent and seep taxa, including vestimentiferan tube worms, vesicomyid clams and bathymodioline mussels (reviewed by Little & Vrijenhoek, 2003:, estimated their origins to be in the later Mesozoic to Cenozoic. Our estimation of the divergence of Bathyacmaea supports these hypotheses about the timing of origination of the vents. However, Geiger & Thacker (2005) have shown that hydrothermal vents were colonized by at least four different vetigastropod lineages (Neomphalidae, Peltospiridae, Lepetodrilidae and Clypeosectidae), during at least three different time periods. Perhaps some of the extant vent taxa invaded vents repeatedly through geological time. In the case of patellogastropods, our results reject the hypothesis that the extant vent taxa are Palaeozoic relics and suggest that the ancestors of the deep-water fauna, including vent taxa, have been elements of the coastal fauna. Nevertheless, the detailed biogeography of the Lepetidae and Pectinodontidac is not clear, because of the limited sampling in the present study. The ecology of these families is poorly known, but Pectinodonta attaches to sunken wood, which mav have aided its distribution, since deep-water circulation may have been highly variable in the Cretaceous (Saltzman & Barron, 1982). The larvae of vent taxa are likely to be transported in hydrothermal plumes (Tyler & Young, 2003). Further research is needed to provide information about the origin and evolution of lepetids and pectinodontids.

Pectinodontidae are a family of deep-sea limpets that live in association with sunken wood, hot vents and cold seeps. The group comprises three genera: Pectinodonta Dall, 1882 (type species P. arcuata Dall, 1882), all species of which live and feed on sunken wood, Bathyacmaea Okutani et al. 1992 (type species B. nipponica Okutani, Tsuchida &
Fijikura,1992) from cold seeps and hot vents (Okutani et al. 1992, 1993; Beck 1996; Sasaki et al. 2003, 2005, 2006), and Serradonta Okutani, Tsuchida & Fijikura, 1992 (type species S. vestimentifericola Okutani, Tsuchida & Fijikura, 1992) from cold seeps (Okutani et al. 1992; Sasaki et al. 2003, 2005). The family is characterised by a distinctive radular morphology, specifically a crossrow comprising a single pair of short (Pectinodonta and Serradonta) or elongate (Bathyacmaea) lateral teeth, each tooth of which shows sutures (Pectinodonta and Serradonta) or structure (Bathyacmaea) suggesting origin through fusion of at least three teeth. The group has long been treated as a subfamily of Acmaeidae (following Pilsbry 1891), but is now firmly established at family level within Lottioidea on anatomical and molecular evidence (Sasaki 1998; Nakano & Ozawa 2007).