278 MANUAL OF THE MOLLUSCA. 
Hipronyx, Defrance. 
Etymology, hippos, a horse, and onyx, a hoof. 
Type, H. cornucopia, Pl. XI., Figs. 20, 21. 
Shell thick, obliquely conical, apex posterior; base shelly, 
with a horse-shoe shaped impression, corresponding to that of 
the adductor muscle. 
Distribution, 13 species. West Indies, Persian Gulf, Philiy - 
pines, Australia, Pacific, West America. 
Fossil, 10 sp. U. chalk—. Britain, France, North America. 
Sub-genus. Amalthea, Schum. A. conica. Like hipponyx, 
but forming no shelly base; surface of attachment worn and 
marked with a cresent-shaped impression. Often occurs on 
living shells, such as the large turbines and turbinelle of the 
Eastern seas. 
Famity XIII.—PATELLIDA. Limpets. 
Shell conical, with the apex turned forwards; muscular im- 
pression horse-shoe shaped, open in front. 
Animal with a distinct head, furnished with tentacles, bear- 
ing eyes at their outer bases; foot as large as the margin of the 
shell; mantle plain or fringed. Respiratory organ in the form 
of one or two branchial plumes, lodged in a cervical cavity; or 
of a series of lamellee surrounding the animal between its foot 
and mantle. Mouth armed with horny upper jaw, and a long 
ribbon-like tongue, furnished with numerous teeth, each con- 
sisting of a pellucid base and an opaque hooked apex. 
The order cyclo-branchiata of.Cuyier included the chitons and 
the limpets, and was characterised by the circular arrangement 
of the branchize. At a comparatively recent period it was ascer- 
tained that some of the patellze (acmea) had a free, cervical 
gill; whilst the chitons exhibited too many peculiarities to 
admit of being associated so closely with them. Professor 
Forbes has very happily suggested that the cyclo-branchiate 
gill of patella is, in reality, a single, long branchial plume, 
originating on the left side of the neck, coiled backwards round 
the foot, and attached throughout its length. This view is con- 
firmed by the circumstance that the gill of the sea-weed limpets 
(nacelle) does not form a complete circle, but ends without 
passing in front of the animal’s head. 
PATELLA, L. Rock limpet. 
Etymology, patella, a dish. 
Synonyms, Helcion Montfort ; Cymba, Adams. 
