ORTHIDiE. 259 



Spec. Char. Semicircular, globose, slightly wider than long, rounded or nearly 

 straight in front ; hinge-line rather less than the width of shell ; ventral valve very 

 convex, and regularly arched in profile, slightly flattened or depressed along the middle ; 

 area moderately wide. Dorsal valve gibbous, sometimes hemispherical, at times de- 

 pressed, but remaining remarkably tumid towards the margins. Surface of both 

 valves covered with numerous fine thread-like strise, increasing in number by bifur- 

 cation and interpolation as they near the margin. In the interior of the ventral valve the 

 saucer-shaped muscular cavity occupies about one third of the width of the shell, and is 

 obscurely divided along the middle by a slightly raised ridge. In the interior of the 

 dorsal valve a prominent median ridge extends from the apex of the umbone to about half 

 the length of the valve, while the deviating brachial processes form two triangular plates, 

 which converge and become united to the upper half of the median ridge or septum, so 

 as to enclose a lozenge-shaped space. The adductor or occlusor muscular impressions lie 

 on either side of the lower half of the median ridge, each pair being again divided by a 

 small oblique ridge. Two specimens measured — 

 Length 11, width 11, depth 9 lines. 



3> O, ,, J, ,5 O ,, 



Obs. This interesting species has been well described by Prof. M'Coy, who states 

 that it occasionally attains one inch in length, and that from the great variation in the 

 proportional depth of the ventral valve in different specimens he is uncertain which of 

 the valves is the deepest, although he entertains little doubt that the dorsal one is so, 

 and that in the adult shell the tumid form and deep short furrow in the rostral half of 

 the dorsal valve distinguish it easily from all he knows. Some specimens that I have 

 seen had their valves evenly convex, but the internal characters presented by the median 

 septum, brachial processes, and shape of muscular scars, at once distinguished 0. turgida 

 from other species of Silurian Orthis. The internal cast of the dorsal valve is also easily 

 recognisable on account of the short but wide slit left by the mesial ridge. 



Position and Locality. 0. turgida appears to be present in the Upper Llandeilo, 

 Caradoc, and Lower Llandovery divisions of the Silurian system. Thus, Mr. Salter quotes 

 it from Craig-y-Glyn in South Wales from the Upper Llandeilo flags ; from the Caradoc at 

 Dolwyddelan, Conway Falls, Bettws-y-Coed, in Caernarvonshire ; at Cerrig-y-Druidion 

 in Denbighshire, and at Llanfair in Montgomeryshire. In Lower Llandovery at Mathy- 

 rafal and Pen-y-Craig. Some large casts occur also in the Cambridge Woodwardian 

 Museum from Craig-y-beri, and Mr. Hughes found it south of Ffairfach Station, both 

 localities being in the Llandeilo flag division. In the Museum of the Geological Society 

 will be found some good examples obtained from the Caradoc at Penllech, Ysputty Evan. 

 Prof. M'Coy mentions, likewise, Llanarmon Fach in Denbighshire, Coed Sion Llangadoc, 

 South Wales, Aber Hirnant, east of Bala, &c. 



In Scotland it has been met with at Penwhapple Glen and Penkill in Ayrshire. 



