212 CAMBRO-SILURIAN FOSSILS. 



This species seems to agree in everything with the little Te- 

 rehratula hamifera (Bar.) (Haidinger's Naturwissenschaftliche 

 Abhandlungen, vol. i. p. 418. t, 20. f. 9), but has the reticular 

 punctation infinitely more minute than he describes that of his 

 species to be (half a millimetre long, or four in a square milli- 

 metre). M. Barrande gives the geological place of his species in 

 Bohemia as "gehort der Quartzitetage (D) an d. h. dem am 

 hochsten gelegenen Theile des untern Silurischen Systems von 

 Bohmen/' and the schists in which ours occurs in such profusion 

 in Britain seem to hold precisely the same place. Its gregarious 

 habits are curiously shown by the circumstance of a fragment of 

 shale four or five inches long and wide from Pen Cerrig having 

 afibrded upwards of 100 specimens now in the Cambridge col- 

 lection, and another mass not much larger from Wellfield having 

 yielded upwards of 70. 



Very common in certain spots in the black shale of Pen Cerrig, 

 Builth, Radnorshire ; and not uncommon in the olive schists of 

 Wellfield, Builth, Radnorshire ; very rare in the olive schists of 

 Pentre Llangynyw near Welchpool, Montgomeryshire. 



{Col. University of Cambridge.) 



Pentamerus microcamerus (M'Coy). 



Ref. & Syn. Spirifer? Icevis, Sow. S. S. t. 21. f. 12 (not Penta- 

 merus lavis, Sow.). 



Sp. Char. Transversely elliptical; receiving valve depressed, gently 

 convex, greatest depth near the beak, which is elevated nearly 

 at right angles, the height being one-eighth the width of the 

 hinge ; entering valve greatly convex ; beaks small, rather pro- 

 minent, reaching to the plane of the lateral margins, with 

 which the low triangular area is nearly parallel, greatest depth 

 slightly behind the middle ; a broad, faintly marked, slightly 

 convex, obsolete mesial ridge, and a few faint, broad, obsolete, 

 irregular lateral radiations ; hinge-line slightly less than the 

 width of the shell, side margins elliptically rounded, front 

 margins very wide, gently convex ; cardinal area nearly sixteen 

 times wider than high ; surface apparently smooth, or with a 

 very minute fibrous longitudinal striation : internal cast of re- 

 ceiving valve showing the broad triangular boss of the fora- 

 men, sKghtly keeled, the mesial septum formed by the junc- 

 tion of its bounding lateral lamellae, only reaching one-third 

 the length of the shell, or little more than equal to the width 

 of the rostral chamber ; cast of entering valve with a very nar- 

 row triangular area in the plane of the lateral margins ; two 

 very slightly diverging dental lamellae, scarcely reaching twice 

 the width of the area (or one- eighth the length of the shell), the 

 exterior edges of which form two more diverging slits, resem- 



