— 29 — 
nalis in valva sinistra subbifidus, in dextra simplex, in utraque 
compresso - triangularis. Margarita cerulescens. 
Altit. 66. — Latit. 115. — Diam. 38. 
H. flumen Usumasinta, ad pagum Balancan Tabascensium. 
71. U. RAVISTELLUS. 
T. parva, depressa, erosa, anterius rotundata, brevis; posterius 
curva, margine supero arcuato , compresso ; extus fuscescens, intus 
plumbea. Dens cardinalis in valva dextra erecta, sepius simplex ; in 
sinistra parva, bilabiata. 
Altit. ?2. — Latit. 38. — Diam. 12. 
H. in lacu Yzabal, reipublice  Guatemalensis. 
78. U. ZERUGINOSUS. 
T. convexo-depressa , ovalis, anterius rotundata, brevis ; posterius 
ovata, parum attenuata ; epidermis castaneo-lutescens ; umibones re- 
tusi, integri, interpolati; dens cardinalis mediocris, in valva sinistra 
duplex, in ambabus conica, angulata, angulis teretibus. Margarita 
lactescens. 
Altit. 32. — Latit. 52. — Diam. 18. 
In rivulo Michol, circa Palenqueanum vicum. 
79. U. OSTREATUS. 
T. convexo-depressa, trigona, subcordata, lamelloso - plicata, 
plicis plus minusve fimbriatis vel irregulariter graniferis, interdum 
evanescentibus, testa omnino rugosa. Epidermis tenax , griseus, ad 
margines lutescens ; dens cardinalis crassus, preelongus, compressus, 
bilabiatus; margarita alba, centro subflavicante vel aureo- purpuras- 
cente. 
Altit. 60. — Latit. 64. — Diam. 29. 
H. flumen Usumasinta. 
SÜ. U. SPHENIOPSIS. 
T. cuneata, tumida, cordata, crassa, tenuisulcata, posterius pli- 
cata, luteo-fuscescens vel virescens, margine inferiori postice si- 
