T. Davidson ^ G. Maw — If. Silurian Rocks of ShropsJiire, 107 



The common or abundant species seem generally to have enjoyed 

 the largest vertical stratigraphical range. For example, Orthis 

 hiloba may be regarded as one of the most abundant species in the 

 Wenlock series, occurring also in diminished numbers in the Lower 

 and Upper Ludlow. The chief horizon, or where it swarms, is the 

 Lower Wenlock Shales or " Buildwas Beds," where we have 

 estimated that upwards of 4500 specimens may be obtained from 

 the washing of a square yard of Buildwas Shale. And from seven 

 tons weight of this shale not less than 25,000 specimens of the shell 

 were obtained, and a vast number more were lost through the wash- 

 ing operations. Orthis elegantida is very abundant both in the 

 Wenlock and Ludlow series, and occurs in no less than nine horizons, 

 but most abundant in the Wenlock Limestone and Shales overlying 

 and underlying it. Orthis hybrida swarms throughout the Wenlock 

 series. 0. Lewisii is likewise found to possess an extended range from 

 the Lower Ludlow down to the Lower Wenlock Shales. The other 

 species of the genus are much rarer and have a limited vertical range. 

 Of the interesting small Streptorhynchus nasiita a single specimen w^as 

 found by the Eev. H. G. Day in the Wenlock Limestone of Benthall 

 Edge. Among the Strophomenidse, St. rhomhoidalis alone enjoys 

 the greatest vertical range, and is at the same time the most abundant 

 species of the group. Leptcena transversalis and especially L, 

 segmentum are very abundant species, while Chonetes lepisma swarms 

 in the shales above the Wenlock Limestone, but has not hitherto 

 been found in Shropshire at any other horizon with the exception of 

 that of the Lower Ludlow. Next to Orthis biloba, Atrypa reticularis 

 is the most abundant species throughout the Wenlock series, and is 

 likewise very common in the Lower Ludlow and Aymestry Lime- 

 stone. Atrypa Barrandi is a very abundant Wenlock species. 

 Meristella tumida possesses an extensive vertical range. Five species 

 of Spirifera have been collected, but of these the small Sp. crispa is 

 the most abundant ; it swarms in the Wenlock Limestone, and 

 especially in the shales that underlie it. Nudeospira piswni is a very 

 abundant species in four horizons of the Wenlock series. Betzia 

 Salteri and especially Retzia Bouchardi are very common species in 

 the Wenlock Limestone and its Shales. Among the Bhynchonellidce, 

 Bh. boreaUs is immensely abundant in the Wenlock Limestone and 

 in its underlying shales, while Rhynchonella Wilsoni is most abun- 

 dant in the Ludlow rocks. 



It is also worthy of remark that while the species belonging to 

 the Clistenterata are prevalent in the Wenlock series, the reverse 

 takes place for the Tretenterata. For example, five species of 

 LinguJa occur in the Ludlow series, while two only have occurred 

 to us out of some fifty thousand specimens of Wenlock Brachiopoda, 

 Lingida Symondi and Orbicnloidea Forbesii being the only abundant 

 species in the Wenlock series, and these apparently almost restricted 

 to the Lower Wenlock Shales. A glance, however, at the table (see 

 over-leaf, pp. 108 and 109), will obviate the necessity of further 

 remarks on this subject. The 81 species recorded in our table belong 

 to some 22 genera. 



