BIVALVIA. 137 



Testa elongatd, securiformi, comprcssiusculd, umbonibus submedianis parvis, margine 

 antico et postico oblique-declivi, basi elliptico curvato ; valvis in medio subdepresso, plicis 

 longitudinalibus magnis distantibus, lunula nulla. 



Shell elongated, subtrigonal or hatchet-shaped, umbones antero-rnesial and small, 

 anterior and posterior borders sloping obliquely downwards, the anterior slopes have the 

 greater angle, lower margin curved elliptically ; the extremities of the valves rounded, and 

 no distinct aperture ; the sides of each of the valves with a large superficial perpendicular 

 mesial depression caused by a few large and distant longitudinal plications. The general 

 figure is more compressed, the umbones more nearly mesial, and the extremities of the 

 valves are more completely closed than is usual in this genus, there is also more or less 

 degree of inequality in the valves. Numerous as are the forms of this genus, we have 

 seen none which are likely to be confounded with the present species ; some of the shorter 

 specimens of Pleuromya elongata, Ag., resemble it in outline only, but the posterior aperture 

 and greater convexity will at once distinguish the species of Agassiz. 



Height, 13 lines ; length, 22 lines; diameter through both the valves, 8 lines. 



Localities. Myacitcs securiformis occurs abundantly in the Cornbrash both of 

 Yorkshire and Wiltshire. 



Mr. Bean has kindly forwarded us a fine specimen from the Great Oolite of Scarborough, 

 which we have figured. 



Myacites decurtatus, Phil. Sp. Tab. XV, fig. 10a, b. 



Syn. Amphidesma decurtatum, Phil. Geol. York., 1, t. 7, f. 11. 

 Pleuromya decurtata, Ag. Etud. Crit. Myes., p. 232. 

 Lutraria decurtata, Goldf. Petref., t. 153, fig. 3. 



Testa ovato-elongatd, umbonibus anticis elevatis, latere antico brevi, abrupte truncaio, 

 postico elongato, aftenuato et hiante ; margine superiori oblique declivi, basi curvato, 

 lateribus plicis longitudinalibus irregularibus. 



Shell ovately elongated, umbones anterior, elevated ; anterior side short, truncated, with 

 a superficial vertical depression ; posterior side elongated and attenuated, superior margin 

 sloping obliquely downwards, the extremity with an aperture of moderate size and elon- 

 gated, lower border curved elliptically ; the sides of the valves with longitudinal irregular 

 plications. 



Compared with its congeners, the elevated umbones, short anterior side, and lengthened 

 attenuated posterior side, will usually serve to distinguish it ; the middle portion of the 

 shell is moderately tumid, the two extremities being somewhat compressed ; the posterior 

 aperture extends both upon the superior and inferior borders ; we have not seen the outer 

 granulated layer of the test. 



Height, 11 lines ; length, 20 lines; diameter through both the valves, 9 lines. 



18 



