300 BRITISH SILURIAN BRACHIOPODA. 



by either a few or many coarse, thread-like, often waving radii, with interspaces between 

 them of greater or lesser breadth ; while a smaller and shorter riblet occupies the middle 

 of the interspace. The entire surface of the shell is also crossed by strong, equidistant, 

 scaly, prominent ridges, or imbricated lines of growth. In the interior of the ventral 

 valve is a prominent tooth on each side of the fissure, and strong dental plates enclose 

 a small saucer-shaped muscular cavity, with raised margin, which is longitudinally 

 divided into two parts by a broadish flattened ridge. In the interior of the dorsal 

 valve the bifid cardinal process is very small, and placed between two deviating raised 

 laminae, on the outer side of which are situated the sockets. Under the cardinal process 

 is a small short septum, which divides posteriorly two laterally notched, curved, and 

 slightly projecting plates, which afforded attachment to the adductor muscle. The 

 whole internal surface of both valves is grooved by irregular radiating striae. 

 Length 14, width 19, depth 6 lines. 



Obs. This is a very variable shell. A small or young specimen only was briefly 

 described in 1839 by J. de C. Sowerby. The adult form was not then known, but 

 was subsequently discovered by myself at Benthall Edge; but not understanding the 

 species then (1847) in all its modifications of shape, I erroneously proposed for the 

 adult form the designation scahrosa. With age the shell thickens considerably, and 

 loses much of that often elevated and pinched-up mesial fold which is so generally 

 present in young and middle-aged specimens (fig. 3). Still, this last character is not 

 even constant in young individuals, and I have before me several examples which show 

 no more appearance of that fold than in Sowerby's original type ; and in some examples 

 the ventral valve is entirely flat (fig. 4), instead of being gently concave or sinuated along 

 the middle. In one specimen (fig. 4) the dorsal valve was even geniculated. The 

 thread-like radiating riblets vary also in number and closeness to a very considerable extent. 

 In 1847, however, I was enabled to completely illustrate the internal characters of this 

 species, as well as that of several others, from excellently well-preserved examples I was 

 fortunate enough to pick up at a time when the refuse-heaps of the large quarry upon 

 the summit of Benthall Edge had not been much explored by collectors. It must 

 also be noted that in some adult examples the thread-like riblets bifurcate (fig. \\ h) ; 

 and that the muscular saucer-shaped depression in the ventral valve, which is usually 

 transverse oval (fig. 12), sometimes becomes lozenge-shaped (fig. 13). The vicinity of the 

 muscular scars presents small granulations, which are well seen in the form of pits in 

 some of the internal casts that occur in the Pentland Hills (fig. 8 b). 



Position and Locality. Strophomena antiqnata is stated in ' Siluria ' to range through 

 the Caradoc, Wenlock, and Lower Ludlow formations. 



From the Caradoc. — M'Coy quotes it from the Coniston Limestone of Water-Head, 

 Lancashire ; the Bala Slates of Blaen-y-Cwm, Nantyre, Glyn Ceiriog, south of Llangollen, 

 Denbighshire ; Bala Slates of Llansantfraid ; but I am not personally acquainted with 

 specimens from these localities, and would urge an attentive search. 



