Ch. xxvn.] aymestry limestone. 553 



monly a micaceous stone, decomposing into soft mud, and contains^ 

 besides the shells just quoted, a Lingula, which is common to it and 

 the " Tilestone " (or Ledbury) beds at the base of the Old lied. The 

 Orthis orbicularis, a round variety of 0. elegantula, is characteristic 

 of the Upper Ludlow ; and the lowest or mudstone beds contaiu ffliyn- 

 chonella navicula (fig. 623), which is common to this bed and the 



Fig. 623. 



Ortlris elegantula.Dalxa. Yar. orbicularis, Athyris (JRhynchonelld) navicula, 

 J. Sow. Delbury. J. Sow. 



Upper Ludlow. Aymestry limestone ; also in 



Upper and Lower Ludlow. 



Lower Ludlow. As usual in the strata of Primary periods older than 

 the coal, the brachiopo clous mollusca greatly outnumber the lamilli- 

 branchiate (see above, p. 543) ; but the latter are by no means unrep- 

 resented. Among other genera, for example, we observe Avicula and 

 Pterinea, Cardiola, Ctenodonta (subgenus of JVucula), Orthonota, and 

 Modiola. 



Some of the Upper Ludlow sandstones are ripple-marked, thus 

 affording evidence of gradual deposition ; and the same may be said 

 of the accompanying fine argillaceous shales which are of great thick- 

 ness, and have been provincially named " mudstones." In some of 

 these shales stems of crinoidea are found in an erect position, having 

 evidently become fossil on the spots where they grew at the bottom 

 of the sea. The facility with which these rocks, when exposed to the 

 weather, are resolved into mud, proves that, notwithstanding their 

 antiquity, they are nearly in the state in which they were first thrown 

 down. 



Lower Ludloiv. — a. Aymestry Limestone. — The next group is a 

 subcrystalline and argillaceous limestone, which is in some places 50 

 feet thick, and distinguished around Aymestry and at Sedgley by 

 the abundance of Pentamerus LCnightii, Sow. (fig. 624),- also found in 

 the Lower Ludlow. This genus of brachiopoda was first found 

 in Silurian strata, and is exclusively a palaeozoic form. The name 

 was derived from ttevts, pente, five, and pepog, 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, 

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

 and De Yerneuil discovered this species dispersed in myriads through 



