206 



LOWER LUDLOW ROCK (CHARACTERISTIC FOSSILS). 



The organic remains of the lower Ludlow, are upon the whole very different from 

 those of the upper Ludlow Rock and Aymestry limestone ; for although a few species 

 of shells are common to the whole formation, this lower division is strongly marked by 

 the presence of many peculiar remains, including two new genera which have not yet 

 been observed in any overlying stratum ; viz. the conchifer Cardiola, and the chambered 

 shell Phragmoceras 1 . (Cardiola interrupta, PI. 8. fig. 7, and Phragmoceras nautileum, 

 PI. 10. figs. 2 and 3.) These with the large Othoceras, 0. filosum, PI. 9. fig. 3; the 

 singularly shaped Orthoceras, 0. pyriforme, PI. 8. figs. 19 and 20; the great Lituites, 

 L. giganteus, PI. 11. fig. 4, and the Graptolites Ludensis, PI. 11. fig. 8, are peculiar and 

 distinguishing fossils. The well-known Trilobites Calymene Blumenbachii, and Asaphus 

 caudatus, Pi. 7. figs. 5 and 6, also abound, but these fossils are equally found in the 

 Wenlock limestone. The small, serrated Graptolite of Linneeus, is very abundant in 

 the ancient rocks of Sweden and Norway. The species above referred to being charac- 

 teristic of the upper Silurian group, and common in the lower Ludlow rock, I have 

 named it G. Ludensis. When first discovered in this country these bodies were supposed 

 to be fucoids, but when examined by botanists they were discarded from the vegetable 

 kingdom. Their true nature was afterwards pointed out to me by an eminent Danish 

 naturalist, Dr. Beck, who after an attentive examination of several varieties found in 

 rocks of the same age, proposes to describe them as zoophytes, bearing an analogy to 

 some of the family of sea pens. 



Here again the analogies and habits of this race of animals, as in the example of the 

 zoophyte of the upper Ludlow rock, (p. 199,) accord completely with the nature of the 

 surrounding sediment ; for as the modern animals to which they are perhaps most 

 nearly allied, live in mud and slimy sediment, so wherever these fossils occur, the rock 

 is a finely levigated mudstone, which from its structure must have been tranquilly de- 

 posited. 



In concluding the account of the Ludlow formation in this district, I may remark, 

 that it is as void of simple minerals or veinstones as it is rich in organic remains. With 

 the exception of crystals of carbonate of lime, which occur in rocks of nearly all ages, 

 I am acquainted with no minerals beyond a very little iron pyrites and some thin strings 

 of galena, one of which I observed in the bed of a brook near Larden in the upper 

 Ludlow, another was detected in the same rock at a depth of sixty feet beneath Ludlow 

 Castle. Besides the small nodules of iron pyrites noticed in the upper beds at Ludford, 

 the same mineral is found occupying the place of the septa of the Orthoceratites which 

 occur in the spheroidal nodules of the lower Ludlow rock. 



Probable thickness of the Ludlow Formation. — The reasons which induced me to hesi- 

 tate in estimating the thickness of the Old Red Sandstone, ought perhaps to prevent 

 my attempting to calculate the exact dimensions of the Ludlow Rocks. We can, how- 



1 These new genera have been named by my friend Mr. Broderip, to whom on all questions connected with 

 the comparison of fossil with recent genera I am under deep obligations. 



