146 



BRITISH SILURIAN BRACHIOPODA. 



Sjjec. Char. Shell varying much in shape, longitudinally (obscurely) ovate or sub- 

 pentagonal, not uncommonly about as Avide as long, or transversely elliptical ; sometimes 

 almost circular, very convex, almost globose. Ventral valve gibbose, greatly arched, much 

 deeper than the opposite valve, vi^ith a broad, slightly elevated mesial fold, commencing 

 at about the middle of the shell, and extending to the front, but sometimes entirely 

 absent, the valve being then uniformly convex ; beak large, ventricose, and usually so 

 much incm'ved as to overlie, and even come into contact with, the umbone of the dorsal 

 valve, so that the triangular fissure beneath the beak is but rarely exposed ; the lateral 

 portions of the beak slope gradually, by a convex curve, to the edge of the fissure. Dorsal 

 valve often almost circular ; posteriorly, evenly convex and gibbous until it reaches about 

 the middle of the valve, when a broad and more or less deepened sinus commences, which 

 extends to the front; lateral margins flexuous, front depressed into a concave wave. 

 Surface of quite young shells sometimes smooth, and marked only by concentric lines of 

 ■growth ; but generally one, two, three, or more obtusely angular ribs commence at about 

 the middle of the valves, and extend along the fold and sinus, the lateral portions 

 remaining smooth. In other specimens, on each of the lateral portions of the valve two 

 or more ribs exist in the vicinity of the margin, and are often of unequal size and width ; 

 while as many as from nine to fifteen are sometimes present on each valve, some of these 

 being occasionally partly divided along the middle or bifurcated. Shell impunctate. 

 In the interior of the ventral valve the dental plates converge, and form a curved, trough- 

 like process, or long, spoon-shaped chamber, while the vertical median septum extends to 

 about one third of the length of the valve, and in its upper part becomes united to the 

 conjoined dental lamellae. In the interior of the dorsal valve two much longer longi- 

 tudinal septa, with a small free space between them, extend about two thirds the length 

 of the valve ; and to these the lengthened socket-walls or plates converge, and, when 

 joined, form two more or less developed plates, inclined outwards, corresjjouding with the 

 trough-like plates of the ventral valve. Three specimens measured — 

 Length 19, breadth 18, depth 16 lines. 

 „ 13, „ 13, „ 9 „ 

 „ 14, „ U, „ 10 „ 

 Obs. This very variable and exceedingly far-spread species has been much more 

 fortunate than many of its congeners ; for, although it has been once or twice placed in 

 the genera Terebratula, Trigonotreta, and Atrypa, almost every author has retained for it 

 the designation galeatus, given to it by Dalman in 1827 ; for I believe that M. Gruene- 

 walt can scarcely be correct while identifying the Fenlamerus Wurmii of F. A. Romer 

 with the Atrypa galeata of Dalman ; and although identified as the Atrypa cassidea of 

 Dalman by Bronn in 1835, the mistake was subsequently corrected by Bronn himself 

 at p. 1288 of his ' Lethaea Geognostica.' Pentamerus galeatus is easily distinguished from 

 P. Knightii both by its external shape, and especially by internal difiercnces, the median 

 septum being shorter and conjoined, supporting but half the V-shaped chamber ; and 



