fe) Unizersity of Michigan 
PLECTOMERUS TRAPEZOIDES Lea, 1831 
Unio crassidens var. a. Lamarck, ’19, p. 71. 
Unio dombeyana Valenciennes, ’27, p. 227, pl. 53, figs. 1, 1°, 1°. 
Unio trapezoides Lea, ’31, p. 69, pl. 3, f. 1. 
Unio interruptus Say, ’31*, p. 525. 
Unio (Bariosta) ponderosus Rafinesque, ’31, p. 2. 
U. trapezoides Lea =U. interruptus Say, Conrad, ’34, p. 72. 
U. crassidens var. a Lam. =U. trapezoides, Lea, Lea, ’34, p. 87 (type 
examined), 
U. trapezoides Lea=U. crassidens var. a Lam.=U. interruptus 
Say, Ferussac, ’35, p. 27. 
U. dombeyana Val. = U. plicata Say, Ferussac, ’35, p: 27 (type lost) ; 
=Q. heros Say var., Vanatta, "10, p: 102. 
Plectomerus crassidens (Lam.) =U. interruptus Say =U. trape- 
zoides Lea, Conrad, ’53, p. 261. 
Bariosta ponderosus Raf. = U. crassidens Lam. = U. trapezoides Lea, 
Frierson, ’14", p. 7. 
Quadrula trapezoides Lea=U. crassidens var. a Lam.=U. dom- 
beyana Val., Simpson, ’14, p. 830. 
Quadrula trapezoides (rea) =U. crassidens Lam., Utterback, ’16, 
p. 88, foot-note; Walker, ’18¢; p..174. 
Our acceptance and adoption of Dr. Pilsbry’s opinion as 
to the recognition of Lamarck’s Unio crassidens carries with 
it the retention of Lea’s name for this species. 
The original description of Unio ponderosus is entirely 
inadequate for the recognition of the species as between Unio 
crassidens Lam. and Unio trapezoides Lea. 
The proportionate dimensions given by Rafinesque for his 
Unio nigra (crassidens Lam.) and his U. ponderosus are the 
same, as they are in fact, in crassidens and trapezoides. The 
difference between the two species being that in crassidens the 
greatest height is at the anterior third of the shell, while in 
trapezoides it is at the posterior third. 
Rafinesque in his generic diagnosis of Bariosta states that 
it has the form of Scalenaria. ‘The only identifiable species of 
that genus is scalenius, of which he gives a figure. While 
there is not much reliance to be placed on his figures for exact- 
