153 



writers had a Mya declivis, which may have been a Thracia, but the iden- 

 tification of it is mingled with doubt. Pennant had a Mya declivis, which 

 he describes as furnishing a favourite dish among the gentry of the Heb- 

 rides ; but this, it will be seen from its habitat, must have been the Mya 

 arenaria. Donovan, Conrad, and Turton had each their Mya declivis, and 

 they have proved to be true Thracia, but all different species, representing 

 respectively T. pubescens, Conradi, and phaseolina. 



The geographical distribution of the Thracia is curious. Two large 

 species, T. convexa and pubescens, are British, but they have only been 

 found at the south-western extremity of our island, on the shores of Devon 

 and Cornwall. Two other large species of similar type are known, one in- 

 habiting the western Mediterranean, the other the Bay of Massachusetts, 

 United States. Lower down on the American continent, at Honduras, 

 California, and St. Thomas's Island, West Indies, is the home of a very 

 distinct type of Thracia, in which the shell is waved obliquely with large 

 plicated wrinkles, T. plicata and magnified, and this type curiously appears 

 in the China Sea in T. granulosa. Passing to below the Equator in the New 

 World, one more presents itself at Brazil, and this strangely enough is of 

 the old European type. In the seas of Europe there is a small species, 

 T. phaseolina, which extends as far north as Sweden ; and there is a spe- 

 cies, T. myopsis, which is confined to the shores of Greenland. All the 

 species of Thracia hitherto spoken of have the outer surface of the shell 

 characterized by a peculiar shagreen sculpture of scabrous granules. In 

 the few southern species of the genus this peculiarity ceases, and the shell 

 partakes of the local structural peculiarities of other allied genera. T. 

 Australica and Novo-Zelandica, from the localities indicated by their 

 names, present the general typical characteristics of the New Zealand 

 Myadora striata; and T. Anatinoides, inhabiting New South Wales, is 

 characterized by the peculiar typical form of the beaked Ana Una of that 

 district. 



In addition to the species already mentioned as British there is a smaller 

 one, T. distorta, which, as the name indicates, has its shell variously dis- 

 torted, from the habit of dwelling in rock. It has very much the appear- 

 ance of a borer, and has been separated as a genus by M. Eleurian de 

 Bellevue under the title of Rupicola. The inequality of the sides of this 

 species is reversed, the posterior being very much the larger instead of the 

 smaller ; and there are two species, T. cuneolus and rudis, the latter from 

 Malacca, presenting the same characters, and probably of similar rock- 

 dwelling habits. 



Species. 



1. Anatinoides, Reeve. 2. Australica, Reeve. 3. conciuna, Gould. 



VOL. II. X 



