217 
EXOGYRA. Say. 
Gen. Char. An unequal valved, unequal sided, 
attached bivalve ; umbones spirally curved to 
one side ; hinge pit curved, nearly linear ; an 
obtuse tooth in the flat, free valve, fits into a 
cavity parallel with the hinge pit in the con- 
vex, attached valve ; one muscular impression 
in each valve. 
A genus established by Mr. Say of Philadelphia* upon 
good grounds ; — we say established by Mr. Say, for the 
imperfect indications given by Faujas j~ and Lamarck 
(who, seeing but one valve of one species, considered it 
an univalve shell, and named it Planospirites ostracina,) 
had been abandoned. It is well distinguished from 
Ostrea by its impressed lateral spire and the consequent 
linear form of the hinge pit, and by the parallel furrow in 
the attached valve which receives the obtuse, compress- 
ed, sometimes striated tooth of the other valve. This 
tooth is an enlargement of that part of one of the edges 
which is often crenated in Oysters. It is distinguished 
from Gryphasa by the same characters of the hinge, by 
the spire being impressed, laterally curved, and very 
apparent in the flat valve, and further by the absence of 
the lobe that forms the distinction between Gryphaea 
and Ostrea. In its general structure it resembles Ostrea, 
* Described by him in the Amer. Journ. of Sci. and Arts, 
vol. ii. p. 43. 
t Faujas, Maestricht, p. 145. Rastellum is a name that has 
been applied to some species; and Faujas has proposed to se- 
parate them and Gryphaea from Ostreae, but this name is not 
applicable. 
X Lam. Syst. 400. This species is very large and smooth, nearly 
related to Ex. haliotoidea ; but we have only one valve, which 
however shows both sides, thus displaying the hinge and muscu- 
lar impression. 
