274 BRITISH SILURIAN BRACIIIOPODA. 



much inflated and projecting above the level of the beak of the ventral valve ; hinge-area 

 narrow, almost perpendicular to the dorsal area ; a wide, slightly elevated, flattened, or 

 rounded mesial fold extends from the extremity of the umbone to the front. In front 

 the margin of the ventral indents that of the dorsal valve. External surface of both 

 valves smooth, marked only by a few concentric lines of growth. 

 Length 12, width 13, depth 6 lines. 



Obs. Although we are unfortunately unacquainted with its interior (some singularly 

 shaped markings are seen on the internal cast of the dorsal valve, fig. 15), the double 

 area and open fissure seem to denote that this shell should be placed in the genus 

 Orthis, and not in either Terehraiula or Sjnrifer, where it has been placed by 

 D'Eichwald and De Verneuil. In 1840 Prof. M'Coy described the shell under the desig- 

 nation of OrtJus (jolea, and it has generally been considered an Orthis by British palae- 

 ontologists. In this species the largest and deepest valve seems to be the dorsal one. 

 Some Irish examples from Portrane have attained thirteen lines in length by eighteen in 

 width. In certain exceptional specimens the mesial fold, in the dorsal valve, becomes 

 obsolete, and is replaced by a deepish longitudinal groove which divides the shell into two 

 equal lobes (fig. 14). 



Position and Locality. This species is said to range from the Upper Llandeilo, 

 through the Caradoc, into the Llandovery. In the Upper Llandeilo (or Llandeilo flags 

 proper) it occurs at Garn, in North Wales. 



In the Caradoc of England and Wales it has been found at Llanfyllin, Montgomery- 

 shire ; and Llaphaiadr, Merionethshire. Mr. Hughes mentions having collected the shell at 

 the River Sevin, Llettyrhyddod, and Cefnrhyddan, in Lower Llandovery ; also at Iron 

 Coldbrook, Llandovery, in Upper Llandovery. Profs. Phillips and Salter quote the 

 species from Mandinam, Goleugoed, Castell Craig, and Gwyddon, in the Llandeilo 

 district. 



In Ireland Orthis insularis abounds in the Caradoc Limestone of the Chair of Kildare, 

 County Kildare; and in the dark limestone and slates at Portrane, Malahide, County 

 Dublin. It is believed also, by Mr. Young, to occur in the Caradoc Limestone of 

 Craig Head Quarry, in Ayrshire. 



On the Continent it has been found in Lower Silurian Limestone in the Island 

 of Dago, according to D'Eichwald, as well as in several other Russian locahties. M. de 

 Verneuil quotes it from Lower Silurian rocks at Bryndlock, in the Gulf of Christiania, in 

 Norway. 



