112 MOLLUSC A FROM THE GREAT OOLITE. 



Testa nucleo elongato, umbonibus parvis anticis depressis, latere antico brevissimo, 

 compresso, postico ventricoso, aperturd ejusdem valde elongatd, margine superiore concavo, 

 tnferiore curvato ; lateribus plicis longitudinalibus magnis et irregularibus. 



Shell with the nucleus elongated, ventricose about the middle portion, and compressed 

 towards the two extremities ; umbones anterior, rather small, and depressed ; posterior 

 aperture of moderate breadth, but very much lengthened upon the superior margin, which 

 is concave ; the inferior margin is curved and nearly parallel to the superior, it has a 

 narrow antero-basal aperture. The sides of the valves have large irregular longitudinal 

 plications, and near to the umbones are some traces of a few radiating lines or costse. 



The aspect of this species is so much compressed from above, and tumid laterally, that 

 the diameter through both the valves exceeds the height of the shell, and exceeds half its 

 length ; there is a superficial depression which extends downwards obliquely to the middle 

 of the lower border, and coincides with the extent of the basal hiatus; the figure altogether 

 is more ventricose and depressed than any other example of Homomya hitherto figured. 

 M. Agassiz appears to have mistaken this species for H. gibbosa, the Lutraria gibbosa of 

 the 'Min. Con.,' tab. 42 and 211 ; but the latter shell differs from it very considerably in 

 figure, it is less depressed, has much larger umbones, is less ventricose in its middle 

 portion, is destitute of the flattening of the anterior side, and likewise of the large longi- 

 tudinal plications of H. Vezelayi. 



We are not aware that the test of M. Vezelayi has ever been found preserved, the 

 prominence of the plication indicates that it was very thin ; we have not seen any traces of 

 the muscular or palleal impressions. 



Localities and position. It is abundant in the clays of the fuller's earth throughout 

 the Cotteswolds, and we have obtained several specimens a little higher in beds of hard 

 sandstone, near to the base of the Great Oolite, on the southern side of Minchinhampton 

 common, associated with several other of the Myadse. 



Myacites crassiusculus. Tab. IX, fig. 3. 



Testa crassd, ovato-elongatd, antice et postice subcompressd, in medio ventricosd, umbonibus 

 anticis subcompressis, latere antico brevi, margine rotundo ; latere postico elongato, aperturd 

 angustd, sed elongatd; margine superiori et inferiori subrectis et parallelis ; lateribus 

 plicis longitudinalibus crebris irregularibus ; area ligamenti magna, lata. 



Shell thick, ovately elongated, umbones anterior, moderately large, and compressed 

 laterally ; anterior side short and compressed, its margin rounded ; posterior side elongated 

 and attenuated, its aperture narrow and elongated upwards, the middle portion of the 

 shell obliquely ventricose ; superior and inferior borders parallel, nearly straight and 

 horizontal ; the cardinal area is large and distinctly circumscribed ; the surface has closely 

 arranged irregular longitudinal plications. The internal surface of the left valve has a 

 curved projecting rib placed a little anterior to and beneath the umbo. 



