SUMMARY OF THE UPPER SILURIAN ROCKS. 



215 



figs, 3 and 5.; Terehratula imbricata and T. cuneata, figs. 12 and 13.; Nerita Haliotis, 

 f. 16, &c. 



Orthocerata occur, but more sparingly than in the Ludlow formation. The Ortho- 

 ceras Brightii, PL 12. f. 21. is a new but rare species. 



Trilobites abound : Asaphus caudatus and Calymene Blumenbachii (PL 7. f. 6, 7 and 8.) 

 which occur in the Lower Ludlow Rock, are the prevailing species. Others, however, 

 are peculiar to the Wenlock formation, and are represented in PL 14, among which 

 are Calymene variolaris, f. 1. ; C. macrophthalma, f. 2. ; Asaphus Stokesii, Nobis, f. 6.; 

 and others including a species of the Isotelus, (Bar Trilobite) and an undescribed genus, 

 which I have named Acidaspis, PL 14. f. 15. The shale or lower part of the formation 

 is characterized by other species of shells, figured in PL 13., the most abundant of 

 which are Productus transversalis, Wahlenberg and Dalm. f. 2. ; Spirifer cardiospermi- 

 forrnis, Dalm. f. 8. ; S. trapezoidalis, f. 14. ; Terebratula brevirostra, f. 12°, ; T. inter- 

 plicata, f. 12 b .; and T. imbricata, PL 12. f. 12.; Asaphus longicaudatus , PL 14. figs. 10 

 and 11.; and Orthoceras attenuatum, PL 13. f. 19. 



I may here observe, that a few shells range through several of the Silurian deposits. 

 One of these, the Atrypa affinis, PL 6. f. 5. {Terebratula affinis Min. Con.), is found in 

 the Aymestry limestone and throughout the Wenlock formation, but it is most abundant 

 in the former. Another species which is very plentiful, Productus depressus (var.) PL 12. 

 f. 2., has the same range; indeed it may occasionally but very rarely be detected in 

 the Caradoc sandstone, its chief habitat being the Wenlock limestone. 



If, however, several individuals are common to two or more formations of the system, 

 the greater number of species are peculiar to each succeeding stratum ; and of the truth 

 of this remark, I shall adduce strong additional evidence in the next chapter, when the 

 contents of the Lower Silurian Rocks are explained. 



The two formations of Ludlow and Wenlock possess so much of a common litholo- 

 gical aspect, and offer such intimate passages from one to the other in the distribution 

 of the organic remains, that they form a very distinct natural subgroup, which I have 

 termed Upper Silurian. The expediency of thus grouping them will be apparent, when 

 these deposits are traced over a more extended area; for whenever the bands of lime- 

 stone thin out, the masses having an uniform argillaceous or mudstone character, are 

 so blended, particularly when they occur together in one mountain mass, (as for example 

 in the Long Mountain, Salop, the adjacent parts of Montgomeryshire, and in Radnor 

 Forest,) that the subdivisions, which are clear in this district, can no longer be de- 

 tected. 



I now proceed to point out the distinctions of other strata, which rising from beneath 

 those above described are termed the Lower Silurian Rocks. 



2 d 2 



