BRITISH SILURIAN BRACHIOPODA. 187 



105. Orthis alternata, Sow. Dav., Sil. Mon., p. 264, PI. XXXI, figs. 1—3, 8 (not 2, 



4, 5, 6) ; Sil. Sup., PI. XIV, figs. 1—6. 



In describing a species we are bound to take as our type the form described by its 

 proposer. Mr. Sowerby describes his species as follows : — " Transversely obovate, finely 

 radiated, one valve convex, the other flat or concave ; radii extremely numerous, of 

 unequal fineness, and increasing in number towards the edge ; hinge-line shorter than 

 the width of the shell ; length 10 lines, width 13 lines. Loc Whittingstow, Soudley, 

 and each flank of the Caradoc. Alt-yr-Anker and Maen Maifod, Lower Lickey Ridge, 

 east flank of Berwyns Mandinan Llandovery " (' Sil. Syst., 1839 '). 



I have in PI. XIV of this Supplement reproduced Sowerby's original figure and 

 given some additional illustrations from specimens taken from the Middle Caradoc at 

 Horderley, and now in the Woodwardian Museum, Cambridge. It is a larger shell 

 than OrtJiis retrorsistria. 



106. Orthis turgida, M'Coy. Dav., Sil. Mon., p. 258, PI. XXXII, figs. 12—20 ; and 



Sup., PI. XIV, figs. 17 to 20. 



This is a good and well-defined species, but it is difficult to procure specimens with 

 both valves in place. Perfectly preserved bivalve individuals show that the ventral valve 

 was deep and more often evenly convex, its beak gently incurved, with a large, triangular 

 area divided in the middle by an open fissure. The dorsal valve, less convex or deep than 

 the ventral one, is longitudinally slightly flattened or depressed along the middle. 

 Salter, in his catalogue of the Cambrian and Silurian Fossils in the Cambridge Museum, 

 p. 61, 1873, when treating of 0. turgida^ says: "One of the most convex of species. 

 The space between the great muscles of the dorsal valve usually occupied by a low ridge 

 is here a sharp one." In Scotland it has been found by Mrs. R. Gray in the 

 Llandeilo at Craighead and Balcletchie. It is quoted by Salter from the Middle Bala or 

 Caradoc group and even from the Upper Llandovery. A specimen in Mrs. R. Gray's 

 collection from the Penkill Limestone of Penwhapple Glen bears much resemblance to 

 those from the Llandeilo ; but this identification will require confirmation. 



