xliv 
SYNOPSIS OF GENERA. 
The relations of Grammysia, Allorisma, Pholadella and Cimitaria, though 
apparent in their exterior ornamentation, require further knowledge of the 
hinge and interior structure before their true affinities can be determined. 
Examples: Pholadella Newberryi, pi. lxxviii, fig. 25. 
Pholadella radiata, pi. lxxviii, figs. 15-21 ; pi. xcvi, fig. 1. 
Phthonia, Hall (Prelim. Notice Lamellibranchiata, 2, p. 70. 1869-70). 
[Type, Cypricardites sectifrons, Conrad.] 
Shell equivalve, very inequilateral, elongate, sub-elliptical. Anterior end 
short and rounded on the margin. Posterior end wider, prolonged, sub-trun¬ 
cate above and rounded below. Beaks sub-anterior, small and appressed. 
Cardinal line straight or slightly arched, ascending from the beak. Umbonal 
slope rounded, more or less distinctly defined. 
Surface marked by fine concentric striae of growth, and by fine or coarser 
radiating striae, which may sometimes become obsolete. 
The valves are united by a narrow, elongate, external ligament (shown in 
fig. 13 of plate lxxviii), but the hinge characters have not been ascertained. 
The anterior muscular scars are small and not deeply impressed; the posterior 
scars are superficial. The pallial line is apparently without sinus. 
Thus far we know the species of this genus only in the Hamilton and Che¬ 
mung groups. 
Six species have been identified, but the individuals of each one are compar¬ 
atively rare. Apparently these forms are not closely allied with any other 
genus among Devonian Lamellibranchs, and for the present constitute a distinct 
group, which are not difficult of recognition.* 
Examples: Phthonia cylindrica, pi. lxxviii, figs. 1-4. 
Phthonia nodicostata, pi. lxxviii, figs. 5-9. 
Phthonia sectifrons, pi. lxxviii, figs. 10-13. 
* Phthonia lirata may be compared with Solemya. 
