116 MOLLUSCA FROM THE GREAT OOLITE. 



The species which approach nearly to the present form, are the Homomya gracilis 

 Agassiz, 'Etud. Grit.,' p. 162, tab. 20, f. 12, and Mya Vezelayi of D'Archiac, 

 'Mem. Soc. Geol. Fr.,' tom 5, pi. 25, fig. 4; but compared with the former shell, the 

 figure is more compressed laterally and less elongated : the concavity of the superior border 

 and larger umbones are other points of distinction. The species described by D'Archiac 

 is very much more ventricose, and the umbones are more nearly terminal ; the posterior 

 aperture is likewise much more considerable. 



Besides these there is another large undescribed species found in the upper division of 

 the Inferior Oolite of the Cotteswolds, which resembles more nearly the present species than 

 either of those before mentioned ; but it is of thrice the linear dimensions, somewhat more 

 elongated, and the superior border is not concave ; the test and ligament, which are very well 

 preserved, enable us to affirm its distinctness both from the Great Oolite species, and from 

 Homomya gracilis, to which perhaps it is still more nearly allied. We possess two 

 specimens, which occurred in the bed of soft shelly Oolite which overlies the Weather- 

 stones : it would, therefore, appear to be very rare. 



Height, 13 lines; longitudinal diameter, 26 lines; diameter through both the valves, 

 12 lines. 

 Locality. Minchinhampton Common. 



MTACITES COMPRESSUS. Tab. XII, fig. 11. 



Testa ovato-rkomboided ; umbonibus prominentibus compressis ; latere antico brevi, 

 compresso, margine ejusdem subrecto declivi ; latere posteriors medocre elongato, convexo, 

 margine truncate ; margine cardinali, subrecto, oblique, declivi ; margine inferiore sinuato ; 

 lateribus sulco lato superficiali ab umbone margine inferiore producto. 



Shell ovately rhomboidal ; umbones prominent and compressed ; anterior side short, 

 its margin nearly straight, sloping obliquely, and somewhat rostrated at the inferior ex- 

 tremity ; posterior side moderately elongated, convex, its margin truncated ; hinge border 

 nearly straight, but sloping obliquely downwards; inferior margin sinuated; the sides 

 with a wide and superficial depression directed from the umbones to the inferior 

 margin. 



The general contour of this species is remarkable for the anterior compression of the 

 valves and of the umbones, which are prominent and very oblique ; the height of the valves 

 is so considerable, that it equals two thirds of the length. The straight anterior slope dis- 

 tinguishes it from Arcomya Couloni, Agassiz, which in other respects it nearly resembles. 

 From our M. tumidus it is separated by the greater height of the valves and oblique slope of 

 the hinge margin, which is also shorter ; the anterior side is likewise more compressed, 

 and its margin straighter. The granulated surface is- not preserved in our specimen. 



Height, 21 lines; length, 33 lines; diameter through both the valves, 16 lines. 



Locality. Minchinhampton Common. 



