144 



BRITISH SILURIAN BRACHIOPODA. 



quaint and defective, shows that the author was acquainted with the internal characters 

 of the shell, and regarded these of sufficient importance for the creation of the excel- 

 lent genus Pentamerus. His Pentamerus Aylesfordii, however, is certainly no more 

 than a transverse variety of the species under description. The ribs on the surface of 

 P. Knightii are at times irregular in width. 



The facility with which the shell separates along the middle is due to the two continuous 

 longitudmal septa in the ventral valve, which are well exhibited in PI. XVI, fig. 2. It 

 is probable that the space left free between the V-shaped plates, or central chamber, 

 was occupied by the digestive organs, while the large lateral spaces were filled up by the 

 ovaries and the spiral arms. 



Of Pentamerus Knightii the largest example I have seen (found by Dr. Lloyd at 

 Mocktree, near Ludlow, and now in the Ludlow Museum) is represented in PI. XVI, 

 P. Knightii is exceedingly common in certain localities, and all our museums possess 

 many excellent examples. In the Swedish island of Far5, in the Baltic, the species does 

 not appear to exist, but is replaced by an allied and equally large form, named P. tenui- 

 striatus, by Walmstedt.^ 



Position and Loeality. P. Knightii occurs rarely in the Wenlock and Upper Ludlow 

 formations ; but it is exceedingly common in the Aymestry limestone at Mocktree, near 

 Ludlow, where I have picked it up on several occasions ; at Aymestry, in Herefordshire, 

 and at Leintwardine, Shropshire. It occurs near Sedgley, near Wolverhampton ; and 

 Prof. Phillips obtained it at Ridge Hill Farm, in the Abberley district, and at Bodenham, 

 in the Woolhope district : all in Aymestry limestone. In the Museum of the Geological 

 Survey in London there are some specimens from the Wenlock limestone of Walsall and 

 Wenlock Edge. In Ireland it occurs east of Perriter's Cove, Dingle, Co, Kerry ; but no 

 specimen has yet been discovered in the Upper Silurian rocks of Scotland. On the 

 Continent, it has been found in Russia, Bohemia, &c. 



for the first specimen of this shell in 1809. I have since received specimens from A. Carlisle, Esq., which 

 have much assisted in illustrating its curious structure, collected by him when on a visit to the gentleman 

 mentioned. A. Aikin, Esq., had observed this formation, and the discovery of the under shells, and 

 mentioned them to me at the meeting of the Geological Society in February, 1812." 



^ Pentamerus Volgicus, De Verneuii, is another large species, very nearly alUed to P. Knightii, found 

 in various localities in the Ural Mountains. At p. 115 of the great work on the 'Geology of Russia and 

 the Ural Mountains,' De Yerneuil details the difi'erences observable between his species and our own 

 P. Knightii ; but I have not yet been able to detect them, as clearly as my friend De Verneuii, in a 

 specimen of the species sent me by Prof. F. Schmidt. The ribs in the Russian specimens of P. Knightii 

 figured by M. V, Gruenewalt are much finer than on any of our British examples. 



