xliv SYNOPSIS OF GENERA. 



The relations of Grammysia, Allnrisma, Pholadella .and Cimifaria, though 

 apparent in their exterior ornamentation, require further knowledge of the 

 hinge and inttyior structure before their true affinities can be determined. 



Examples : Pholadella Newbernji, pi. Ixxviii, fig. 25. 



Pholadella radiata, pi. Ixxviii, figs. 15-21 ; pl. 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 striaa of growth, and by fine or coarser 

 ratliating striag, which may sometimes become obsolete. 



The valves are united by a narrow, elongate, external ligament (shown in 

 fig. 13 of plate Ixxviii), 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, pl. Ixxviii, figs. 1-4. 

 Phthonia nodicostata, pl. Ixxviii, figs. 5-9. 

 Phthonia sectifrons, pl. Ixxviii, figs. 10-13, 



* Phthmtia lirata may be compared with Solemya. 



