150 Couthouy's Monograph on 
THRACIA SIMILIS. 
Plate IV. fig. 3 
testa ovato-o Shion! Sipe albida vel giderek; subdiaphanå 
inequilaterali, latere deos longiore, truncato et subcompresso, an- 
marginem infero-posticam decurr rente ; car- 
dine Frac Vd P irren valvà utraque ligamento externé pro om- 
inulo; intus albà, impressionibus muscularibus anticé elongatis, quasi 
clavatis, posticé rotundatis ; ; impressio pallii posticé valdé excavatà ; 
an ossiculum? 
Shell dai icu: of a greyish 
white, or ash color, ferruginous at the margins, mode- 
rately convex, covered with numerous pretty regular, 
concentric striz of increment, and feeling rough to the 
touch, in consequence of being covered with minute 
granulations, most conspicuous near the margins and on 
the truncated end; a very thin, grayish epidermis, strongly 
adherent, and hardly to be distinguished from the shell 
itself, extends over the whole surface ; somewhat ine- 
quivalve, the right valve a little the larger and project- 
ing beyond the left; beaks moderately prominent, in- 
clined a little backwards, the summit of the left one 
acute, and fitting into that of the right valve, which is 
excavated to receive it ; slightly gaping at both extrem- 
ities, inequilateral; the posterior extremity, which in 
some individuals is almost twice the length of the an- 
terior, is broadly truncated, a little narrowed, and that 
portion between the dorsal margin and the obtuse 
wave, which extends from the beaks to the lower ex- 
tremity, is somewhat compressed ; the anterior extrem- 
ity is oval, the basal margin regularly but not very 
strongly curved; cardinal edge delicate, ligamentary 
