ORTHlDiE. 267 



Obs. This species was well described and figured by Mr. Salter in 1848 ; but I 

 believe that distinguished palseontologist was mistaken when he wrote that the valves 

 were not crossed by any lines of growth, for in a well-preserved example now before me 

 numerous close equidistant concentric lines, or slightly raised ridges, intersect the 

 radiating ribs. He adds likewise that "the sKght depth of the central channel, sometimes 

 hardly visible, readily distinguishes our shell from 0. vespertilio, which in the fasciculation 

 of the striae and general form it resembles ; but the ribs in that are more numerous, 

 closer, and broader than the interstices." Prof. M'Coy further observes — " The great 

 diiference which I have registered in the number and general characters of the radiating 

 ridges may be traced in such a way among the Scottish specimens as leaves no doubt of 

 the specific identity of the forms, some of the specimens, with more numerous ridges, 

 having them nearly uniform in size, while some of the specimens with the more distant 

 ridges have them more or less fasciculated, as in the extreme forms above alluded to ; in all 

 cases, however, the species seems very distinct by the narrow, very sharply defined, round 

 linear ridges, and smooth, flat, wide interspaces ; the casts are strongly marked with 

 radiating ridges round the margin." 



Position and Locality. According to Mr. Salter this species occurs in the 

 Llandeilo at Bogang in Ayrshire, as well as in the Caradoc or Bala beds at Craig Head, 

 near Girvan. I have seen it very abundant in rocks at Mullock Hill Quarry, near Girvan, 

 in beds attributed to the Llandovery, for along with it occurred Meristella angustifrons, 

 Atrypa hemispharica, and other species belonging to that period. It has also been found 

 at Knockdollian, three miles from Ballintrae, in Ayrshire, as well as at Ardmillan, and in 

 the limestones of the Stincher River. Prof. M'Coy mentions the shell in the Bala 

 Limestone of Llandeilo, Caermarthenshire (finely marked specimens), and in the limestone 

 of Trownscoed Gaerfawr. 



Orthis patera, Salter, MS. PL XXX, figs. 1—8. 



Spec. Char. Semicircular, wider than long ; hinge-line less than the width of the shell. 

 Ventral valve moderately and evenly convex, area narrow ; dorsal valve almost flat, area 

 narroAv ; surface of both valves covered with very fine thread-like radiating striae, here 

 and there crossed by concentric lines of growth. In the interior of ventral valve the 

 saucer-shaped muscular cavity is large and laterally margined by projecting edges, and is 

 likewise divided longitudinally by a broadish flattened ridge. In the interior of the 

 dorsal valve, on either side of the cardinal process, are two diverging projecting brachial 

 processes, grooved along the middle; from under the cardinal process a short median ridge 

 extends along the bottom of the valve and separates the adductor impressions. Two 

 specimens measured — 



