56 University of Michigan 
diameter are 60% and 40%, respectively). Thus the name 
mastus Lea is the oldest available one. 
The later names given by Lea and claimed by Ortmann as 
possible synonyms (cylindrellus and corvunculus) need not be 
considered here. Their validity depends upon taxonomic con- 
siderations, and these two forms require additional study 
before their standing can be settled. 
Genus CoNRADILLA Ortmann, 1921 
Type: Unio celatus Conrad. 
Conradilla Ortmann, Naut., XXXIV, 1921, p. go. 
CONRADILLA C&LATA (Conrad), 1834 
Type locality: Tennessee, Elk, and Flint rivers. 
Unio (Lemiox) rimosus Rafinesque, ’31, p. 3. 
Umo celatus Conrad, ’34, p. 338, pl. 1, f. 2; ’34, p. 20, pl. 13, f. 4. 
Micromya celata (Con.), Simpson, ’14, p. 34. 
Lemiox rimosus (Raf.) =U. celatus Con., Frierson, ’14, p. 7. 
Lemiox rimosus (Raf.), Ortmann, ’16, p. 39; '18, p. 574. 
Micromya celata (Con.), Walker, 18°, p. 4, 18°, p. 185 (new generic 
name should be proposed). 
Frierson has identified U. rimosus with U. celatus chiefly 
on the strength of the word “rimose”’ in the description, which 
is intended to describe the sculpture of the shell. Walker 
objects to this. Although this word may be taken as giving 
a good description of this feature of the shell, other points in 
the description do not fit. 
Rafinesque (731, p. 4) makes the following explicit state- 
ment of his theory of comparative measurements: “The com- 
parative proportions of the length, breadth, diameter and axis 
of the Unios and other bivalve shells having been misunder- 
stood by some, it may be needful to state that my formula is 
a kind of abbreviation of a longer exposition. Thus, when I 
say, length one-half, diameter one-third, axis one-fourth of 
the breadth, | meant to say and must be understood to state 
