82 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



Oxfordian Oolites ; for although it occurs in the Cornbrash of Yorkshire and Wiltshire, it 

 is rarely found in a lower position than the Kelloway Rock. Roemer records his Lutraria 

 sinuosa in the Lower Coral Rag of Heersthum ; Agassiz places his Plcuromya recurva in 

 the Terrain a Chailles of Chamsol, in the department of Doubs. 



MYACITES SINISTRA, Agassiz, sp. Tab. XXXV, figs. 17, 17 a. 



ARCOMYA SINISTRA, Agassiz. t. Grit. Myes., p. I/O, tab. 9, figs. 1 3, et tab. 9', 



figs. 1013. 

 PANOP.&A SINISTRA, If Orb. Prodr., i, p. 273. 



Oppel. Juraformation, p. 480. 



Testa ovato-elongata antice attenuata, postice convexa producta, margine hiaitfe, 

 umbonibus subcompressis, depressis, antemedianis, laterc antico obligue-declivi, lunula 

 concavo, margine superior* subhorizontali margine inferiore subrccto; valms lateribus 

 plicis irregularibm crebis lonf/itudinalibm, ct sidco supcrficiali antemediano oblique-declivi. 

 Nucleus glaber. 



Shell ovately elongated, with the sides of the valves rather flattened ; anterior side 

 attenuated, its margin sloping obliquely downwards ; lunule concave ; posterior side more 

 convex and lengthened, its superior border nearly horizontal ; the posterior extremity is 

 somewhat rounded, with an aperture moderately large ; the umbones are depressed, and 

 somewhat compressed laterally ; they are placed a little posterior to the anterior third of 

 the shell, and there is a slight sulcation, which proceeds from them obliquely forwards and 

 downwards towards the lower border; the inferior margin is lengthened and nearly 

 straight ; the surface of the test has numerous irregular and rather delicate longitudinal 

 plications ; the granules over the greater portion of the valves are so minute and crowded 

 that they cannot be traced to form connecting lines, but towards the sides they are larger, 

 more distantly arranged, and distinctly linear ; the test upon the anterior side is of 

 moderate thickness, posteriorly it is much thinner; the nucleus is smooth, and exhibits 

 the adductor and pallial scars. 



Length, 2 inches ; height, 1 inch ; diameter through both the valves, f inch ; but our 

 specimen is imperfect, and appears to have lost about 2 lines in length at the posterior 

 extremity. 



The more depressed umbones, the anterior attenuation, and the nearly horizontal 

 figure of the superior border, will serve to distinguish it from all the varieties of Pleuromya 

 elongata, Ag., to which it bears some resemblance. 



The Arcomya sinistra ofQuenstedt, 'Der Jura,' p. 451, tab. Ixii, fig. 2, from the higher 

 stage of the Inferior Oolite, occurs also in the same position in the vicinity of Cheltenham ; 

 it is, however, distinct from Myacites sinistra. Some varieties of Myacites decurtatum 

 approach to it in the general figure, but are readily distinguishable when the granulated 



