284 BRITISH SILURIAN BRACHIOPODA. 



Obs. The above specific description is taken from p. 120 of my ' Monograpli of 

 Carboniferous Brachiopods/ and it applies equally well to the Silurian As to the Carbo- 

 niferous representatives of the species. I have here given all the principal synonyms and 

 references to the Silurian form ; and these can be completed by a reference to those 

 recorded in the Carboniferous and Devonian Monographs. In PI. XXXIX will be seen 

 the principal modifications which the shell assumed during the Silurian period. 



When formerly describing this species it escaped my notice that we were indebted to 

 C. F. Wilckens (17C9) for the first description, name, and figure of this important 

 species. The fact is recorded by Wahlenberg at p. QQ of the eighth volume of the ' Nova 

 Acta Acad. Upsaliensis,' published in 1821, but was overlooked by myself and almost 

 every other pala3ontologist. During the Silurian period, and in some other specimens, the 

 extension of the lateral portions of the shell was very considerable, as will be seen in 

 figs. 1, 10, 11, 15. Some Swedish examples of the interior of the ventral valve (fig. IC) 

 have exhibited in a most beautiful manner the impression of the coils of the oral arms 

 affixed to the interior of the dorsal valve ; and I found this to occur likewise in some of 

 our British examples (fig. 14"). Certain young shells are also remarkably square (fig. 6), 

 while others have their front scooped out into three projecting lobes (fig. 4). Lejjtcsna 

 tenuistriaia, J. de C. Sow., has also by many palseontologists been considered to be 

 simply a variation in shape of Strophomena rhomboidalis ; the striation is often irregular, 

 but in some specimens the interspace between the more prominent thread-like radii is 

 large and well marked, one or two smaller ribs or striae filling up the interspace. Another 

 variety (PI. XLIV, fig. 1) has been found in the Caradoc of Craig-Head Quarry, near 

 Girvan, in Ayrshire. 



Po-ntion and Locality. This remarkable species ranges through the Caradoc, Lower 

 and Upper Llandovery, Wenlock, and Ludlow formations. It passes up into the 

 Devonian, and continues during the Carboniferous period. It is present in a great many 

 localities. We will enumerate a few of the principal ones. 



In the Caradoc .• Carnedd Dafydd, Llyn Idwal, Snowdon, Bettws-y-Coed, &c., in 

 Caernarvonshire; south-east of Cerrig-y-Druidion, Pont Hafod, Gynfor, and Cefn Coch, 

 south-west and south of Llangollen, Denbighshire; Llanfyllin, Llanwyddyn, Meifod, 

 Gaerfawr, Guilsfield, Welshpool, Montgomeryshire ; east, west and south-east of Bala Lake, 

 Brynbedwog, &c., in Merionethshire ; Bird's Hill, Llandeilo ; Keisley, Westmoreland, and 

 Helms Gill Dent, in Coniston Limestone. 



In Lower Llandovery .• at Quaker's Burial Ground, Welshpool ; Cefn, near 

 Buttington ; Mathyrafal Haverfordwest ; Cvvar Mawr, Cilgwyn ; Blaencwm, Llando- 

 very ; Llettyrhyddan, &c. 



In 'Uj)per Llandovery : at Builth ; Penlan, Llandovery ; Chirbury ; Norbury ; 

 Malverns ; May Hill ; west of Crochley ; Tortworth. 



In the Woolhope beds of the Malverns. 



In the Wenlock Limestone and Shales of Dudley, Linley's Lime- kilns near Walsall, 



