ee 
/" 
em 

Parr IL. Secr. ii. § 2] SILURIAN. 681 
which has been prolific in Lower Ludlow fossils, particularly in star-fishes 
and eurypterid crustaceans, a fragment of the fish Scaphaspis (Pteraspis) 
ludensis was discovered in 1859. This is the earliest trace of vertebrate 
life yet detected. I1t is interesting to note that this fish does not stand 
low in the scale of organization, but has affinities with our modern 
sturgeon. 
(b.) Aymestry Limestone—a dark grey somewhat earthy concretionary 
limestone in beds from 1 to 5 feet thick. Where at its thickest it forms 
a conspicuous feature, rising above the soft and denuded Lower Ludlow 
shales, and, owing to the easily removable nature of some fuller’s earth 
on which it lies, it has here and there been dislocated by large landslips. 
Jt is still more inconstant than the Wenlock limestone. Though well 
developed at Aymestry it soon dies away into bands of calcareous 
nodules, which finally disappear, and-the lower and upper divisions of 
- the Ludlow group then come together. The organic remains at present 
known number 53 genera and 84 species, which for the most part are 
identical with Wenlock forms. It is evident that the organisms which 
flourished so abundantly in the clear water in which the Wenlock lime- 
stone was accumulated continued to live outside the area of deposit of the 
Lower Ludlow rock and reappeared in that area when the conditions for 
their existence there returned during the deposition of the Aymestry 
limestone. The most characteristic fossil of the latter rock is the 
Pentamerus Knightii; other common forms are Rhynchonella Wilson, 
Lingula Lewisii, Strophomena euglypha, Bellerophon dilatatus, Ptlerinea 
Sowerbyz, with many of the same shells, corals, and trilobites found in the 
Wenlock limestone. Indeed, as Murchison has pointed out, except in 
the less number of species and the occurrence of some of the shells more 
characteristic of the Upper Ludlow zone, there is not much paleonto- 
logical distinction between the two limestones.! 
(c.) Upper Ludlow Rock.—In the original Silurian district described 
by Murchison, the Aymestry ‘limestone is covered by a calcareous shelly 
band full of Bhynchonella navicula, sometimes 30 or 40 feet thick. This 
layer is succeeded by grey sandy shale or mudstone, often weathering 
into concretions, as in the Lower Ludlow zone, and assuming externally 
the same rusty-brown or greyish olive-green hue. Its harder beds are 
quarried for building stone; but the general character of the deposit, 
‘like that of the argillaceous portions of the Upper Silurian formations as 
a whole in the typical district of Siluria, is soft, incoherent, and 
crumbling, easily decomposing once more into the original mud, and 
presenting in this’ respect a contrast to the hard, fissile, and often slaty 
shales of the Lower Silurian series. Many of the sandstone beds are 
crowded with ripple-marks, rill-marks, and annelid-trails, indicative of 
the shallow littoral waters in which they were deposited. One of the 
uppermost sandstones is termed the ‘“ Fucoid Bed,” from the number of 
its cylindrical sea-weed-like stems. It likewise contains numerous 
inverted pyramidal bodies, which are believed to be casts of the cavities 
made in the muddy sand by the rotatory movement of crinoids rooted 
and half buried in the micaceous mud.? At the top of the Upper 
Iudlow Rock near the town of Ludlow, a brown layer occurs from a 
quarter of an inch to three or four inches in thickness, full of fragments 
of fish, Pterygotus, and shells. This layer, termed the ‘‘ Ludlow Bone- 
1 Siluria, p. 130. = 2 Op. cit. p. 133. 
