244- Mr. Williamson on the Limestones fotmd 



cimens of Axinus obscurus, and an AvicuJa, which will be no- 

 ticed hereafter. 



At Bedford the niagnesian limestone reappears under a 

 form very similar to that at CoUyhurst, but has been far more 

 extensively exposed. It has for some time been quarried for 

 burning, but at present the pit is filled with water, so that the 

 fine section there presented from necessity cannot now be ex- 

 amined. As at CoUyhurst, it consists of a series of red clays, 

 with thin beds of limestone. The limestones are in numerous 

 thin seams, rarely more than twelve inches in thickness. To- 

 wards the upper part they become still thinner, varying gene- 

 rally from one to four inches, and not forming continuous 

 seams, but layers of flattened concretions, on the surface of which 

 are found abundance of fossils. These limestones are variable 

 in colour ; the lower and thicker seams are of a greenish yellow 

 colour, and the thinner ones more tinged with the red sub- 

 stance which has given the colour to the separating marles. 



Sect. III. Fossil Remains. 



The fossils of the magnesian limestone are neither nume- 

 rous nor of peculiar interest ; they chiefly consist of Axi?ius 

 obscurus, Avicula, Crassina ?, Trochus, and several varieties of 

 minute turbinated shells, probably species of Turbo. The 

 Axinus obscurus of CoUyhurst differs only from those of Bed- 

 ford and Yorkshire in having a regular series of concentric 

 striae on its external surface. These I have observed in 

 impressions of some from the above places ; but neither the 

 granular limestone of Bedford, nor the calcareous shelly marie 

 of Yorkshire, is so well calculated to preserve the delicate 

 characters of the shells as the fine clay of CoUyhurst. The 

 Axinus is found in every stage of growth, and at both localities, 

 with the exception of one single specimen from Bedford, con- 

 sists entirely of single valves. From this it would appear 

 that the shells had been gradually covered up, the ligaments 

 being destroyed before they were buried and protected from 

 decomposition and disunion of the valves. 



I am uncertain whether the Avicula be an undescribed spe- 

 sies or not, but rather suppose it is. It approaches closely to 

 Avicula socialis of Sowerby, which has induced some to con- 

 sider the stratum as identical with the muschelkalk. I believe 

 Professor Phillips is of opinion that the species is undescribed. 

 The Trochus, which is a small and beautifully granulated one, 

 as well as the minute turbinated shells, are certainly new, al- 

 though some of them may be referred to by Professor Sedg- 

 wick, in his paper on the magnesian limestone of Yorkshire, 

 without either figure or description. Two or three smaller 



