THRACIA. 



Plate I. 



Genus THKACIA, Leach. 



Testa ovata vel oblouga, plus minus inaquilateralis et ince- 

 quivalvis, nunc depressa, nunc gibbosa, albida, plerum- 

 que granoso-scabrosa, epidermide tenui partim induta, 

 latere postico s&pissime longiore subangulato, scepe 

 truncato, plus minus hiante, antico rotundato, valvd 

 sinistra interdum subplanulatd. Canto fulcro hori- 

 zontali varie incrassalo in utrdque valvd, interne exca- 

 vato, ligamentum recipiente, cavitate ligamentario an- 

 tice aperto, ossiculo crescentiformi libera annexo ; liga- 

 mentum partim externum. Impressio pallii profundi; 

 imbuta ; musculorum parvce, dissimilar es. 



Shell ovate or oblong, more or less inequilateral and in- 

 equivalve, sometimes depressed, sometimes gibbous, 

 whitish, generally granosely scabrous, partially co- 

 vered with a thin epidermis, posterior side generally 

 the longer, slightly angled, often truncated, more or 

 less gaping, anterior side rounded, left valve some- 

 times rather flat. Hinge with a horizontal variously 

 thickened fulcrum in each valve, internal excavated 

 receiving the ligament, ligamentary cavity anteriorly 

 open, with a free, crescent-shaped ossicle annexed ; 

 ligament partially external. Impression of the mantle 

 deeply marked ; impressions of the muscles small, 

 dissimilar. 



Thracia is one of a family of genera grouped together by 

 M. Deshayes under the title of Osteodesmata, in which the 

 hinge-cartilage is partly external and partly internal, the in- 

 ner portion being contained within a shelly cavity, which 

 at its fullest development is projected into a jutting out, 

 spoon-shaped process. It is in Anatina and Periploma that 

 the spoon process is most conspicuous. In Thracia it takes 

 the form of a thickened marginal fulcrum in each valve, 

 and when the valves are closed and the fulcra are in con- 

 tact, with the ligament between them, an opening appears 

 internally at the anterior end ; and we have the authority 

 of observers for stating that at this point a loose crescent- 

 shaped ossicle is annexed. This bony appendage or ossicle 

 I have never seen. Notwithstanding all the care with 

 which Mr. Cuming has collected by far the finest series 

 of Thraciee known, he has never obtained the ossicle. Be- 

 ing free, it is, doubtless, lost when the animal is extracted ; 

 and so variously modified is the hinge of the different 

 species of Thracia, it is by no means certain that they all 

 have it. 



It was by an English naturalist, Dr. Leach, that the 

 genus was originally founded ; two of the largest and 

 most characteristic species being denizens of the shores 

 of Cornwall and Devon. The earlier British writers had 

 a Mi/a declivis, which may have been a Thracia, but the 



identification of it is encompassed with doubt. Pennant 

 had a Mya declivis, which he describes as furnishing a 

 favourite dish among the gentry of the Hebrides: but this, 

 it will be seen, must have been a Mya arenaria. Donovan, 

 Conrad, and Turton had each their Mya declivis, all diffe- 

 rent species, T. pubescens, Conradi, and phaseolina ; the 

 name declivis therefore falls to the ground. The genus, as 

 introduced by Leach, was quickly adopted on the continent, 

 and we now present it with twenty-two species. 



In addition to the two large Thracia of our own south- 

 western coast we have a large species of similar type on 

 either side of us, one, T. corbuloides, inhabiting the west- 

 ern Mediterranean, and the other, T. Conradi, inhabiting 

 the Bay of Massachusetts. Lower down on the American 

 continent, at Honduras, California, and St. Thomas's Is- 

 land, is the home of a very distinct type, in which the 

 shell is waved obliquely with large plicated wrinkles, T.pli- 

 cala and magnifica; and this type curiously appears in the 

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

 the New World, one more species presents itself to the 

 collector, at Brazil, and this is of the old European type. 

 In the seas of Europe there is a small species, T. pha- 

 seolina, which extends as far north as Sweden; and there 

 is a species, T. myopsis, which is confined to Greenland. 

 All the 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 northern generic feature ceases, and the shell 

 partakes of the peculiarities of the locality. T. Australica 

 and Novo-Zelandica, from the places respectively indicated 

 by their names, present the general typical characteristics 

 of the curious New Zealand genus Myadora ; and in the 

 T. anatinoides of Sydney we have a species partaking of 

 the tvpe of the beaked Anatina of the same continent. 



A British species, T. dislorta, of rock-dwelling habits, 

 and having very much the appearance of a borer, has 

 been separated as a genus by M. Fleurian de Bellevue, 

 under the title of Rupicola ; and the shell is really very 

 distinct. It will be seen, on reference to the figure, that 

 the inequality of the sides is reversed, the posterior being 

 very much the larger side instead of being the smaller. 

 Two more species, T. cuneolus and rudis, come into the 

 same subdivision. 



Species 1. (Mus. Cuming.) 



Thhacia Corbuloides. Thr. testa ovato-oblongd, valde 

 intequilaterali, gibbosiusculd, tenui, inflata, postice 

 compresse biangulatd, subquadrato-truncatd, antice tu- 

 mido-rotundatd, valvis subrugosis, irregularibus ; un- 

 dique minute scabrosd, albida. 



November, 1859. 



