Ca. XXYII. j 



AYMESTRY LIMESTONE. 



2. Aymestry limestone. — The next group is a subcrystalline and 

 argillaceous limestone, which is in some places 50 feet thick, and dis- 

 tinguished around Aymestry by the abundance of Pentamerus Knightii, 

 Sow. (fig. 572), also found in the Lower Ludlow. This genus of brachi- 



Fig. 572. 



Pentamerus Knightii, Sow. Aymestry. Half nat. size. 

 a. View of both valves united. 

 o. Longitudinal section through both valves, showing the central plates 



6epta. 



Fig. 5T3. 



opoda was first found in Silurian strata, and is exclusively a paleozoic 

 form. The name was derived from tfsvrs, pente, five, and ^s^og, meros, 

 a part, because both valves are divided by a central septum, making four 

 chambers, and in one valve the septum itself contains a small chamber, 

 making five. The size of these septa is enormous compared with those 

 of any other brachiopod shell ; and they must nearly have divided the 

 animal into two equal halves ; but they are, nevertheless, of the same 

 nature as the septa or plates which are found in the interior of Spirifer, 

 Tcrebratula, and many other shells of this order. Messrs. Murchison 

 and De Verneuil discovered this species dispersed in 

 myriads through a white limestone of Upper Silurian 

 age, on the banks of the Is, on the eastern flank of 

 the Urals in Russia, and a similar species is frequent 

 in Sweden. 



Three other abundant shells in the Aymestry lime- 

 stone are, 1st, Lingula Lewisii (fig. 573) ; 2d, 

 Rhynchonella Wilsoni, Sow. (fig. 574), which is also 

 common to the Lower Ludlow and Wenlock lime- 

 stone ; 3d, Atrypa reticularis, Lin. (fig. 575), which 

 has a very wide range, being found in every part of 

 the Silurian system, even in the upper portion of. the 

 Llandeilo flags. 



Fig. 574. 



LingvZa Lewisii, 



J. Sow. 



Abberley Hills, 



Rhynchonella (Terebratula) Wilsoni, Sow. Aymestry. 



28 



