710 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



with an elongated downward and backward slope from the hinge-mar- 

 gin ; the latter margin nearly straight, and occupying about two-thirds 

 the whole length of the shell. Surface marked by only the ordinary 

 lines and coarser imbrications of growth, but usually the angular umboual 

 ridge is cut across by three or four short, distinct ridges and corre- 

 sponding furrows, extending obliquely inward and backward, being 

 scarcely perceptible in front of the umbonal ridge, and becoming obso- 

 lete before reaching the postero-dorsal margin, or at least only pro. 

 ducing slight sinuosities upon it. 



Length 58 millimeters; height from base to umbones 34 millimeters; 

 thickness 28 millimeters. 



The elongate subtriangular outline, prominent and angular umbonal 

 ridges, and broad, flattened dorsum of this species, are features that 

 readily separate it from all other known forms, and, together with the 

 seven other species associated with it (mentioned in the next descrip- 

 tion), show an extent and diversity of differentiation among these 

 earlier species of Unionidce that is hardly surpassed at the present day. 



Locality and position. — Upper part of the Laramie Group ; Black 

 Battes Station, Union Pacific Eailroad, Wyoming. 



Unio aldrichi (sp. uov.). 



Shell of medium size, transversely elongate, approximately oblong in 

 marginal outline, a little higher posteriorly than anteriorly; moderately 

 gibbous, especially along the umbonal ridge, where the shell is thickest ; 

 test moderately thick, becoming much so in old shells ; beaks placed 

 nearly one-third the length of the shell from the front margin, incurved, 

 broad, but not very prominent, although the flattened umbo is raised 

 above the level of the hinge-line ; umbonal ridge prominent, subangu- 

 lar; postero-dorsal portion of the shell behind this ridge compressed, 

 sometimes subalate ; front portion of the shell moderately gibbous, and 

 between this and the umbonal ridge the sides are distinctly flattened . 

 anterior margin regularly, but somewhat narrowly, rounded down to the 

 basal margin, which is nearly straight along the middle ; postero-basal 

 margin somewhat narrowly rounded, and extended upward and back- 

 ward to the postero-dorsal margin ; the latter margin sometimes trun- 

 cated obliquely downward and backward, and sometimes so rounded as 

 to give a more nearly square truncation to the posterior end of the shell ; 

 hinge-line long and straight. Surface marked only by the ordinary 

 lines of growth, except all that portion which lies behind the umbonai 

 ridge. This portion is marked by numerous sharply-raised, irregular 

 lines or narrow ridges, with the intervening spaces wider than the 

 ridges themselves, which, beginning almost imperceptibly just behind 

 the umbonal ridge, extend backward with a greater or less uj)ward 

 curve to the dorsal and posterior borders. These raised ridges increase 

 in number with the growth of the shell, in very small part by implanta- 

 tion, but mainly by bifurcation. They usually constitute a conspicuous 



