SYNOPSIS OF GENERA. xxxi 



genus Chanomya, 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 ; Grammjsia has also a priority of seventeen years over Chanomya. 

 A reference to the type of Ch&nomya (C. Leavenworthemis) will also show the 

 marked differences between that form and those here referred to the genus 

 Grammysia. Chanomya 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 O. cingulata, 

 Hisinger sp., O. 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 bisulcata, pi. liv, figs. 1-16 ; pi. 

 ivi, fig. 1 ; pi. xciii, fig. 25. 

 {Section b. obsoleta) Grammysia obsoleta, pi. lix, figs. 21-27. 

 {Section c. undulata) Grammysia alveata, pi. Ivii, figs. 1, 2; pi. Ix, 



figs. 1-11. 

 {Section d. elongata) Grammysia communis, pi. Ixi, 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 



