ACEPHALA. 429 
are its best diagnostic marks. ‘These are usually concealed by a thick 
crust of black earth. It is said to come from streams in the vicinity 
of chalybeate springs. It varies considerably in form. 
Figures 542, 542a, 5426, three views of the shell; 542c, the 
animal. 
Unto prorucus (Gould). 
T. transversa, elongato-ovalis, ventricosa, epidermide _fusco-castaneo, con- 
centrice undato-striato, radiatim obsolete plicato induta ; margine ven- 
trali viz arcuato; margine dorsali sub-recto ; latere antico rotundato ; 
latere postico oblique truncato, ad apicem obtuso; apicibus ante- 
medianis, elevatis, erosis ; dentibus cardinalibus obliquis, elongatis, 
compressis ; dentibus lateralibus remotis, sub-arcuatis: margarita 
hivido-albida. 
Unio profugus, Goutp; Proceed. Boston Soc. Nat. Hist., iii. 295. 
Expedition Shells, 89. 
SHELL solid, transversely sub-oval, sub-equilateral, moderately tu- 
mid; anterior end narrowest, elliptically rounded; ventral edge 
slightly curved; posterior dorsal margin curved about equally with 
the basal, the downward slope a little oblique, the tip obtuse; beaks 
two-fifths from the anterior end, considerably elevated, eroded ; poste- 
rior umbonal slope rather full. Surface rather coarsely undulated by 
the stages of increment, and with faint, radiating strie, sometimes 
amounting to undulations on the disk, which is rather flat; epidermis 
dark dirty olive, shining, sometimes wrought into radiations of ripple- 
like curves on the disk. Nacre bluish-white, not brilliant, punctate, 
with bronzed blotching posteriorly, well thickened at the ventral por- 
tion. Hinge well developed; cardinal tooth obliquely transverse, of 
two unequal lobes in the right valve, single in the left; a deep rounded 
sinus between the cardinal and the lateral tooth, which is well deve- 
loped, a little curved, divided a little way at tip: muscular impression 
well marked. 
Length two and three-fourths inches; height an inch and a half; 
width seven-eighths of an inch. 
108 
