SYNOPSIS OF GENERA. 
XXXI 
genus Cho&nomya, overlooking the fact that the type of Grammysia is a New 
York species, and has been made the basis of comparison of all the species 
published in the Preliminary Notice of the Lamellibranchiata and in the present 
volume ; Grammy sia has also a priority of seventeen years over Chmornya. 
A reference to the type of Ch&nomya. ( C . Leavenworthensis ) will also show the 
marked differences between that form and those here referred to the genus 
Grammysia. Chmnomya has the valves gaping behind as in the recent genus 
Mya, the pallial line is sinuate and the beaks are small and appressed. 
Mr. J. W. Salter (Mem. Geol. Surv. Gt. Britain, pp. 359-361, 1848) considers 
Grammysia as a sub-genus of Orthonota and that Allorisma is a synonym of the 
latter genus. In accordance with this, he describes the species 0. cingulata , 
Hisinger sp., 0. triangulata , Salter, and 0. extrasulcata, Salter, and notes their 
relationship with the species of Grammysia , as defined by De Verneuil. 
The genus Grammysia seems to be well-founded in nature, and is quite 
distinct from Orthonota, although with our present knowledge it is difficult to 
clearly separate Grammysia from Allorisma, as now recognized, on account of 
the numerous intermediate species which seem to unite the two extremes of 
variation. 
Examples: ( Section a. cingulata) Grammysia hisulcata, pi. liv, figs. 1-16; pi. 
lvi, fig. 1; pi. xciii, fig. 25. 
(Section b. obsoleta ) Grammysia obsoleta, pi. lix, figs. 21-27. 
{Section c. undulatd) Grammysia alveata, pi. lvii, figs. 1, 2; pi. lx, 
figs. 1-11. 
{Section d. elongata) Grammysia communis, pi. lxi, figs. 24-28; pi. 
xciii, fig. 21. 
For convenience of reference and description, the species described in this 
volume have been arranged in the four natural groups as above indicated. 
The first group represents the typical form of Grammysia, while the last group 
includes species which have sometimes been referred to Allorisma, and may per¬ 
haps with propriety be included in that genus. 
The generic description of Allorisma (King, 1844), as redefined (King,- Perm. 
Foss., p. 196, 1850), is here given: “ Equivalved: inequilateral, the posterior 
side being the longest: in general slightly gaping. Valves granulated on the 
