Cx XXVII. j 



AYMESTEY LBIESTONE. 



4V> 



2. Ay7nestry 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 Knightk^ 

 Sow. (fig. 5*72), also found in the Lower Ludlow. This genus of brachi- 



Fig. 572. 



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

 a. View of both valves united. 

 &. Longitudinal section through both valves, showing the central plates j' septa. 



Fig. 6T3. 



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

 form. The name was derived from -TrsvTi', pente, five, and ixs^ocr, 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 S-pirifei^ 

 Tcrehratula^ 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 Ui'als 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, 

 Bhynclionella Wilsoni^ Sow. (fig. 574), which is also 

 common to the Lower Ludlow and Wenlock lime- 

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

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

 the Silurian system, even in the upper portion of the 



Lingula Lewisii, 



J. Sow. 



Abberlcy Hills. 



Llandeilo flags. 



Fig. 574. 



Ehynchonella {Terelratuln) Wilsoni, Sow. Aymestry. 

 28 



