SCUISTODESM US 939 
I have seen only a single shell of this form (No. 126472 of 
the U. S. Nat. Museum) and it is so different from radulosa 
that if I were positive that it was normal I would give it 
specific rank. It is badly eroded, but is very differently shaped, 
has much less strongly marked sculpture and is more tawny 
than typical shells. With it is a normal younger specimen of 
radulosa from the same locality. 
Genus SCHISTODESMUS Simpson, 1goo. 
Shistodesmus SIMPSON, Syn., 1900, p. 803. 
Shell rather solid, triangular, inflated, truncate above in 
front, somewhat swollen just behind the center of the base, 
pointed behind; beaks high; beak sculpture not seen; surface 
thrown up into strong, very wide, concentric ridges, one or two 
with each season’s growth, and pinched up in the central part 
to form a radiate row of compressed knobs or spines; epi- 
dermis shining, greenish-yellow or brownish, having mar- 
velously delicate, concentric, microscopic lire, often painted 
with a few elegant, broken, green rays; there is one large, 
high, triangular pseudocardinal in the left valve under the 
beak, a deep, triangular pit in front of it, with an erect, radial 
lamellar tooth in its center and a high, lamellar, curved tooth 
in front of the pit, parallel with the edge of the shell connected 
with the central tooth. A heavy triangular bifid or trifid pseu- 
docardinal in the right valve stands close to the anterior edge 
of the shell and is separated from it by a deep, compressed 
pit ; behind this tooth, deep down at its base, is a narrow shelf, 
and back of this the entire hinge plate is cut out in a deep 
sinus to the beak; two laterals in each valve; cavity of the 
beak in left valve deep, somewhat compressed; in the right 
entirely opened by the sinus; muscle scars small, deep, smooth : 
pallial line with a sinus at its posterior end; a beautiful, gran- 
ularly striate callus is developed behind the laterals; nacre 
finely, often bifurcately, radiate striate outside the pallial line. 
Animal unknown. 
Type, Unio lamprevanus Baird and Adams. 
