BIVALVIA. 119 



Testa elongatd, convexd ; umbonibus mediums, urea magna, marginibus depressis, lateribus 

 plicis longitudinulibus magnets, striis longitudinulibus dense impressis. 



Shell elongated convex ; umbones mesial, area large, its margins faintly marked, the 

 sides with very large regular plications, which are impressed with very fine densely ar- 

 ranged longitudinal striae. 



The length of the posterior side slightly exceeds the other ; its extremity is slightly 

 curved upwards, it is rarely preserved or perfectly represented upon the internal moulds ; 

 the lines of radiating tubercles cannot be distinguished upon the moulds. Height 9 lines ; 

 lateral diameter, 22 lines; diameter through both the valves, 8 lines. 



Locality. Minchinhampton. 



Goniomya, Ag. 



Shell very thin, cylindrical and ventricose, or ovate and flattened, gaping at both the 

 extremities, more especially the posterior extremity ; anterior extremity rounded, posterior 

 truncated ; umbones mesial or a little anterior to the middle of the valves, contiguous and 

 not very prominent ; costae large and curved, their anterior portions are directed obliquely 

 backwards towards the inferior border, the posterior portions are directed in a similar 

 manner forwards, so that the extremities of the costae meet each other near to the middle 

 of the shell at an angle more or less acute. The costae are crossed and indented by 

 closely-arranged concentric plications. The substance of the test has two layers, of which 

 the outer one is semi-corneous, and is furnished with minute tubercles which are arranged 

 in lines radiating from the umbones. Hinge edentulous ; muscular impressions faintly 

 marked ; ligament external. 



Goniomya litterata, Sow. sp. Tab. XI, fig. 3. 



Goniomya litterata, Agassiz. Etud. Crit., p. 18, t. 16, f. 13-16. 

 Mya litterata, Sow. Min. Con., t. 224, fig. 1. 

 Lysiaxassa litterata, Gold/. Petref., t. 15-1, f. 8. 



Testa ovato-ehngatd, convexd; umbonibus ante-medianis, margine cardinali subhorizontuli 

 aut concavd, margine antico oblique declivi; costis unticis angustis subundatis; posticis magnis 

 curvatis ultimis evanescentibus ; plicis concentricis crebris decussatis; angulo costurum ucuto, 

 obliquo margine postico producto; margine inferiore subrecto. 



Shell ovately elongated, convex, umbones placed anterior to the middle of the valves, 

 superior margin elongated, nearly horizontal, or even slightly concave, anterior margin 

 sloping obliquely, inferior margin nearly straight ; costae anteriorly narrow, nearly straight, 

 and slightly undulated, posterior costae larger, curved but become obscure towards the 

 extremity of the series ; the costae are decussated with closely arranged regular concentric 

 plications ; costal angle acute, and directed obliquely towards the infero-posterior 

 border. 



