ADDENDA. 215 



pes-anseris, Bronn, ibid., tab. xi, fig. 8 ; Lyrodon Kefersteinii, Miinster, Goldf,, 

 '■ Petrefacta,' tab. cxxxvi, fig. 2. There is nothing analogous to this section in the genus 

 Trlyonia. 



The second group, for the most part also subtrigonal in figure, has the surface 

 destitute of ornamentation, or has only longitudinal plications ; it is without any clearly 

 defined posteal area, or has an imperfect marginal angle. Examples : Lyrodon ovatum, 

 Goldfuss, 'Petrefacta,' tab. cxxxv, fig. 11 ; Lyrodon Iceviyatum, Goldf., ibid., tab. cxxxv, 

 fig. 12; also Lyrodon simplex, Goldf., ibid., tab. cxxxv, fig. 14. This group in its 

 surface characters has afiinities with some of the Triyonia glabra, and more especially 

 with Triyonia Linyonensis, Dum. 



The third group has longitudinal costse and a posteal area, which is separated from 

 the other portion of the valve by a marginal carina ; the area has also oblique costellae ; 

 it thus approaches to the Triyonia Costatce in various features, and more especially by 

 the differences exhibited by the opposite valves of the same species ; differences, however, 

 which are wholly distinct from those exhibited by the Jurassic Triyonia of the allied 

 section. The single British species Myophoria postera belongs to this third group. 

 Other examples are Lyrodon lineatum, Miinster, * Petrefacta,' tab. cxxxvi, fig. 4 ; and 

 Myophoria Goldf tissii, Bronn, ' Lethaea,' tab. xi, fig. 7. This group is allied to the 

 Triyonia costata, yet possesses an unerring distinctive feature ; the Jurassic Triyonia 

 have the marginal carina of the right valve larger than that of the other, and is never 

 divided, as in the Myojjhoria, into two or three smaller carina, which cross the dorsal 

 portion of the valve. 



In venturing to propose the foregoing sectional divisions for Myophoria^ I would 

 offer them only as exemplifying the present knowledge of that genus, at the same time 

 avowing the possibility that future discoveries in Triassic palaeontology may tend to 

 modify very materially the divisional groups here proposed. 



Myophoria postera, Quenst.^ sp. PI. XLI, figs. 6, 7, 8, 9 ; 6 a, 7 a, 8 <a', 9 a. 



Tbigonia postera, Quenstedt. Der Jura, tab. i, figs. 3 — 6, p. 28, 1857. 

 Myophoria — Moore. Quart. Journ. Geol. Soc, p. 507, pi. xvi, figs. 8, 9, 10, 



1861. 



Diaynostic Characters. — Shell very convex at the divisional angle of the valve, 

 subtrigonal ; umbones rather depressed, directed anteriorly, forming a buccal excavation 

 anteally ; the lower margin is lengthened and curved elliptically. The left valve has an 

 elevated marginal carina, which is denticulated more or less prominently upon its lower 

 half ; it is bounded anteally by a strongly defined ante-carinal groove ; there is no 

 distinct median or inner carina. The area, which is steep and wide, has a series of 



