S T R ^ A. 



Genus OSTR.EA, Linntem. 

 Tesia hivalris, unimmatlnsa, foliacea, ad .titxart, rniicli<i.'', 

 radicesque per valvam inferiorem adhmrens, valde 

 irregularis ; cardine edentulo, marij'me lineari; li'ja- 

 mento suhextemo fiiper tirmm triijdnam utrilque 

 valvd Inminato. 

 Shell bivalve, unimustnilar, toliiiceous, iullioriiig to 

 stones, shells, and roots by the lower valve, very 

 irregular ; hinge toothless, with linear margin ; 

 ligament partly external, laminated upon a trigo- 

 nal area in each valve. 

 East, west, in the tro])it'al, temperate, arctie, and 

 antarctic zones, -wherever there are sea-coasts, and 

 even in deep seas, oysters are to bo found. And while 

 the genus is widely diffused, individual species seem to 

 be spread over very large areas. If some forms existing 

 in Austraha, America, Chiloe, &c., variously named, 

 be not really varieties of our native Oatrcea edulis, 

 certainly many forms, distributed from the Adriatic 

 to the Mediterranean sea, are clearly referable to that 

 very variable species. As must be the case with 

 attached bivalves, the difficulty of distinguisliing species 

 is very considerable, owing to the form of each indivi- 

 dual depending so largely upon the object and mode of 

 attachment. Our figures of Ostraa ohdis present a 

 striking example of this, and to the divergent forms 

 given many others might be added, such as the pon- 

 derous 0. Iiippoptis, 0. cristata, and very probably, 

 O. lamellosa, which are forms of the same. Among 

 the most interesting of new species are some described 

 towards the latter part of the mon^Jtrraph. from the 



NoveiJil 



collection of Sir Davi( 

 pally in the Mauritius, 

 Barclay — the latter of 



1 Barclay — shells taken princi- 

 aml the islets Diego Garcia and 

 quite recent formation. 



Species 1. (Mus. Sowerby.) 

 OsTR.EA PRisMATiCA. Ost. testd lonfjitudinaliter elon- 

 ijatu-ovdtd, crassd, sijuamiferd,fu.tcd, squamis irlaU 

 jiiniori divaricatim sfriatis ; intuK iridesceuti, propS 

 mdn/iiiciii fusco-pvrpurascenti unifasdato mit 

 nibvhitn : eiratvice nia.rimo ; cardine lata, niffres- 

 ceiiti, l('iiijiliii!iiii(/iti r tiltcnuatd, profunde siilcato, 

 sidris iJi riilriiiii iiifcriinrin and ccntrali, in oalvam 

 siipei-iori'in diiahns talcralibus : valvd in/erivri 

 concavd, fuliaced. 

 The pnissiATic Ostk^ea. Shell longitudinally elongato- 

 ovate, thick, squamiferous, brown, scales in the 

 young state divergently striated ; iridescent 

 within, near the margin single-banded or clouded 

 with purple-brown ; scar very large ; hinge 

 blackish, longitudinally attenuated, deeply 

 grooved, grooves in the lower valve one central, 

 and in the tipper valve two lateral ; lower valve 

 concave, foliaceous. 

 Gkay. Annals of Philosophy. 1825. 

 Ostrava iridcscens. Gray, Conch. Misc. Ost. f 6, 7. 

 Hab. Centrid America. 



A young specimen of this shell, in the collection of 

 Mr. Taylor, is invested with dark -purplish scales whicli 

 are divergently striated. The older sjiecimens retain 

 no traces of this, their surface being worn away as 

 they attain any thickness. 



