272 BRITISH SILURIAN BRACHIOPODA. 



four or five lines from the beak, the others branch irregularly lower down once or twice ; 

 lateral ribs varying from six to fifteen within three or four lines from the beak, branching 

 irregularly, some into two, others into four, with age ; surface crossed towards the margin 

 with sharp striae of growth. This variety does not seem to have been noticed either in 

 Russia or America, yet it is extremely common in our old rocks ; many show a difference, 

 in the muscular impressions in the dorsal valve, from the variety Lyiix in having the 

 posterior pair much smaller than the anterior, and obhquely elongated, so that the division 

 between them and the anterior pair is no longer a horizontal line forming a cross-shaped 

 mark ; other specimens, however, more distinctly preserved, seem to have the usual internal 

 characters along with the divided ribs. So very variable are the ribs in size and number that 

 extreme specimens which I have noted might be referred on the one side apparently to 

 the Sp. terehraUiliformis, M'Coy, and on the other to the Sp. ovata, M'Coy.'' 



In PI. XXXVIII I have endeavoured to represent specimens illustrating the principal 

 modifications in shape presented by this species as occurring in Great Britain. In 

 England it does not attain moi-e than about half the size of some large American examples. 



Position and Locality/. Its vertical range seems to have been considerable. In ' Siluria' 

 Orthis biforata is stated to occur in the Llandeilo, Caradoc, Llandovery, and Wenlock form- 

 ations ; and in all these, with the exception of the last, I have found it to occur abundantly. 

 We can only name some of the principal localities, for to enumerate them all would require 

 more space than can be given to the subject. I do not, however, know where it has been 

 met with in the Llandeilo formation. 



In the Caradoc it occurs at Bettws-y-Coed, in Caernarvonshire, and south-east of 

 Cerrig-y-Druidion, south of Llangollen, Denbighshire ; at Llanfyllin, Alt-yr-Ankcr, Meifod, 

 Llanwddyn, and Pen-y-Craig, Llanwddyn, Montgomeryshire; west of Bala Lake, 

 Pont-y-Glyn, Bryn Melyn Quarry and Bryn Bedwog, near Bala, Merionethshire, Diff'wys, 

 west of Corwen, Denbighshire. In the schists of High Haume, Dalton in Purness, 

 Lancashire ; also at Keisley, Westmoreland, &c. 



In Lower Llandovery, at Cefn-Rhyddan, Llandovery ; Mathyrafal, Pen-y-Craig, &c. 



In Upper Llandovery, at Pentrefflymru, Prcsteign, Bogmine, &c. 



In the Wenlock Limestone and Shales at Dudley, and at the Rushall Canal near 

 Walsall, Stafibrdshire. 



In Scotland it occurs in Caradoc Limestone at Craig Head Quarry, two miles north 

 of Girvan, in Ayrshire. 



In Ireland it abounds in many places, such as the Chair of Kildare, Carrictadaggan 

 and Ballybro, Ballyeale, County Wexford ; Bally Daniel Gorey, Grangegeeth, County 

 Meath, &;c., all in Lower Silurian rocks. 



The variety fissicostata, M'Coy, abounds in the Caradoc at half a mile south of 

 Brynbebedney, near Balaj Cwmgwenan Uchaf, and Ketch Bridge, near Llanfylhn ; in the 

 schists of Peniarlh, Meifod, and Bala Sandstone of Alt-yr-Anker, Montgomeryshire ; 

 at Bala, Bryn Melyn, Aber Hirnant, Bala Schists of Gelli Grin, Merionethshire. In the 



