308 Mr. A. Adams on new Genera and Species of 
menti concentricis ad marginibus validioribus instructa, latere. 
postico vix flexuoso ; lunula linea impressa circumcincta. 
Alt. 8 lin., lat. 9 lin. 
Hab. Off Quelpaart, 48 fathoms. 
The valve from which the above description is taken is fresh 
and in fine condition. It is white, spotted irregularly with pale 
orange, and the surface is smooth and shining. 
Genus Iacra, H. & A. Adams. 
Shell ovate, thin, pellucid, closed at both sides; valves with 
the surface divaricately sculptured. Hinge with an oblique 
spoon-shaped cartilage-pit and a small anterior primary tooth in 
each valve. Lateral teeth two, stronger in night valve. 
Tacra Japonica, A. Ad. 
I. testa ovata, ineequilaterali, tenui, pellucida, nitida, convexa, latere 
antico latiore rotundato, postico angustiore et subproducto ; valvis 
utrinque concentrice striatis, medio sulcis radiantibus obliquis or- 
natis, margine ventrali arcuato, integro ; sinu palliali magno, lato, 
profundo. 
Hab. Kino-O-Sima. 
Smaller, more vitreous and pellucid, and with coarser sculp- 
ture, than J. Seychellarum, A. Ad. 
Genus TuErcoponta, A. Adams. 
Testa oblonga, zequivalvis, valde inzequilateralis, concentrice sulcata, 
latere antico breviore, postico longiore. Cardo (in valva sinistra) 
dentibus duobus divaricatis, cam dente theczeformi prominente inter- 
posito, munitus; dente antico crasso, arcuato, antice tuberculiformi ; 
dente postico lamelliformi, tenui; dente laterali antico nullo, postico 
conspicuo elongato. FPallii linea simplex, valde impressa et radiatim 
sulcata ; cicatricula musculari antica triangulari profunda, postica 
elongato-ovata. 
In form and general character this genus seems to approach 
the fossil genus Goodalliopsis lately proposed by MM. Raincourt 
and Munier-Chalmas for a shell from the “ Calcaire grossier ” 
of Fercourt. In that genus, however, there is simply a trian- 
gular space between the cardinal teeth, and the teeth themselves 
are nearly similar; while in Thecodonta there is a cup-like fold 
projecting into the cavity of the shell, and the primary teeth 
are very dissimilar, the anterior being of a semilunar form, with 
a tubercular head at the theciform process, and the posterior 
being lamelliform and nearly parallel with the hinge-margin. 
There is only one lateral tooth, which is situated posteriorly. I 
have drawn up my description from left valves only, so that 
further information is required concerning the genus, especially 
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