278 MANUAL OF THE MOLLTTSCA. 



HiPPOiTS'X, Defrance. 



Etymology, hippos, a horse, and onyx, a hoof. 



Type, H. cornucopia, PL XI., Pigs. 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, Philij - 

 pines, Australia, Pacific, "West America. 



Fossil, 10 sp. XT. chalk — . Britain, France, Korth 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 turbinell^o of the 

 Eastern seas. 



Pamily XIII. — Patellid^. 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. Eespiratory organ in the form 

 of one or two branchial plumes, lodged in a cervical cavity ; or 

 of a series of lamellae 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 cydo-hranchiata of Cuvier included the chitons and 

 the limpets, and was characterised by the circular arrangement 

 of the branchise. At a comparatively recent period it was ascer- 

 tained that some of the patellse {acmcea) had a free, cervical 

 gill ; whilst the chitons exhibited too many peculiarities to 

 admit of being associated so closely with them. Professor 

 Porbes 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 

 {nacelloe) does not form a complete circle, but ends without 

 passing in front of the animal's head. 



Patella, L. Pock limpet. 



Etymology, patella, a dish. 



Synonyms, Helcion Montfort ; Cymba, Adams. 



