80 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



dimensions, with a lengthened general form, compressed anterior side, and large longitudinal 

 plications over the whole of the surface ; it appears to be comparatively rare, and belongs 

 to the upper stage of the Inferior Oolite. 



THRACIA AMYGDALOIDEA, Lye. Tab. XLIII, fig. 4. 



Testa convexa, elongata, umbonibus dcpressis submedianis, lutcre antico prodticfo, ro- 

 tundato ; postico subcompresso, attenuate, basi curvato, plicis longitudinalibus panels, 

 levitcr instructis. 



Shell elongated, convex ; umbones postero-mesial, depressed ; anterior side produced, 

 its margin curved elliptically ; posterior side rather compressed and attenuated, its superior 

 border slightly excavated ; the base is nearly straight ; the surface has a few faintly 

 marked, longitudinal plications of growth. 



Compared with other examples of the genus, the length and the convexity are con- 

 siderable ; the umbones are likewise much depressed, obtuse, and but little conspicuous ; 

 the posterior angle is only distinguishable near to the umbones ; the posterior extremity is 

 slightly truncated. The height only very slight exceeds half the length. 



Geological Position and Locality. Associated with valves of Myacites calceiformis in 

 flaggy, argillaceous Oolite, upon the western border of Minchinhampton Common, at the 

 lower boundary line of the Great Oolite ; a single specimen. 



MYACITES CALCEIFORMIS, Phil., sp. Part II, Tab. XI, fig. 2 ; et Tab. XLII, figs. 1, 1 a. 



As this shell possesses considerable variability of figure, another example is given from 

 the Cornbrash of the Yorkshire Coast. In the former description (p. 1 14, line 8), these 

 words should be erased " in the upper beds of the Inferior Oolite." An examination of 

 numerous Yorkshire specimens has proved that they were all obtained in the Cornbrash, 

 including the original specimen figured in the ' Geology of Yorkshire,' which was erroneously 

 placed with the Inferior Oolite fossils, and figured with them in pi. xi of that work. 

 The Cornbrash specimens have the test with its granulated tegument well preserved, but 

 usually the fossil has undergone some compression or distortion. The former figure, 

 Plate XI, fig. 2, represented a Minchinhampton specimen from the base of the Great 

 Oolite. An Inferior Oolite shell frequently mistaken for Myacites calceiformis occurs 

 only in the form of casts ; it is more gibbose, with larger, more elevated umbones, the 

 posterior side being much shorter and more attenuated. As the casts are common, and 

 these distinctive characters are persistent, there can be no doubt that it must be dis- 

 tinguished from the species of Professor Phillips. Authorities generally have followed the 

 ' Geology of Yorkshire,' and placed Myacites calceiformis in the Inferior Oolite, and Dr. 



