314 BRITISH SILURIAN BRACHIOPODA. 



and formed of numerous radiating ridges and furrows distinct and strong to the very 

 end, while in Strophomena compressa, the nearest ally, the upper edge alone of the 

 muscles is visible, the muscles themselves hardly producing any mark/' This last state- 

 ment, however, is exaggerated ; for, as may be seen from the drawing of St. compressa in 

 PI. XLVI, fig. 10, these marks are often clearly defined, although not so strongly grooved 

 as in the shell under description. " The interior, too, is smooth, and not granulated in lines 

 as in St. compressa ; the vascular impressions, when present, radiate from the central ridge 

 a little below the base of the muscles, and curve outwards and upwards to the margin, 

 forking two or three times ; the space between the top one and the muscles pitted ; 

 the striae are only visible round the edge interiorly. The hinge-teeth in the dorsal valve 

 are large and prominent " (he should have said the bijid cardinal process, as this part 

 does not serve as a tooth, but afibrds a point of attachment to the attenuated extremity 

 of the divaricator or cardinal muscle^), " and supported on a thick central rib or plate, 



which divides the muscles ; there is also an accessory tooth, long and 



narrow, on each side of this pair." Here, again, Mr. Salter is wrong, for the narrow 

 plate, which he calls a tooth, has nothing to do with the articulation of the hinge further 

 than to limit on one side the socket-wall. 



According to Prof. M'Coy, Orthis alternata. Sow. (' Sil. Syst.,' pi. xix, fig. 6), is a 

 synonym ; but the generality of Palaeontologists have retained them as distinct forms. 



Position and Locality. Strophomena expansa occurs in the Carodac and Lower 

 Llandovery. In the former it is abundant at Meifod, near Welshpool ; Hope Bowlder, 

 Soudley, near Church Stretton ; near Llanfyllin ; Harnage, south-east of Shrewsbury ; 

 Port Rhiwaedog, in the Hirnant Valley, near Bala ; Carnedd Dafydd ; Bettws-y-Coed ; 

 Dolwyddelan ; Pwllheli, Caernarvonshire. South-east of Cerrig-y-Druidion ; Llangollen, 

 Denbighshire. Cader Dinmael, Montgomeryshire; east, west, and south-east of Bala 

 Lake ; Llanrhaiadr, Merionethshire, &c. 



In the Lower Llandovery at Mathyrafal ; quarter of a mile south-east of Cwmrhyddan ; 

 Cwar Mawr, Cdgyn Road, &c. 



In Ireland at Desertcreat and Bardahassiagh, Tyrone ; Grangegeeth ; Lombay 

 Island; Knockmahon, Tramore, &c. I believe it also occurs in rocks of the Caradoc 

 period near Girvan, in Ayrshire. 



Many more English and Welsh localities are enumerated by Prof. M'Coy ; but, as I 

 have not seen specimens from those localities, I merely refer the reader to p. 218 of the 



1 In his admirable memoir on the ' Organization of the Brachiopoda,' Mr. Hancock thus describes the 

 position of this muscle : — " The divaricator (or ' cardinal muscles' of some authors) arise from the ventral 

 valve, one on each side, a little in advance of, and close to the united bases of the occlusors (adductors). 

 These extremities are large, fleshy, and of a reddish-yellow colour; they rapidly diminish in size, and 

 attain the appearance of white, glistening tendons ; and having the intestine between thera converge as they 

 pass backwards and upwards towards the cardinal process, into which their superior, attenuated extre- 

 mities are inserted in close continuity." 



