222 CAMBRO-SILURIAN FOSSILS. 



reaching half the length of the shell, not divided by any me- 

 sial septum, but having one or two strong sulci on each side ; 

 margin finely sulcated by the external striae, faint extensions 

 of which reach a variable distance towards the beak : cast of 

 entering valve very tumid, nearly smooth, deeply slit towards 

 the beak by the very deep narrow sulcus, left by a thick me- 

 sial septum, reaching half the length of the shell, and to which 

 the lamellae of the very short, thick, cardinal teeth seem to 

 converge, forming the mesial septum by their union, the wide 

 triangular boss of the foramen slit by a slender rostral tooth. 

 Length of small specimen 9 lines, proportional length of en- 

 tering valve /q%, width -^q^q, depth of receiving valve -^^^q to 

 y^o^Q, depth of entering valve y^^^ to y^^^^. (Length occasion- 

 ally 1 inch.) 



From the great variation in the proportional depth of the en- 

 tering valve in different specimens, I am uncertain which of the 

 valves is the deepest, although I have little doubt the entering 

 one is so. I have seen small specimens confounded with O. ele- 

 gantula, from which the characteristic turgid appearance from 

 the tumidity of the sides, larger mesial striae, &c. distinguish it ; 

 in the adult state the tumid form and deep short furrow in the 

 rostral half of the entering valve distinguish it easily from all I 

 know. 



Extremely common in the schists of Craig y beri, Llanarmon 

 fach, Denbighshire ; schists of Coed Sion, Llangadoc, S. Wales ; 

 very common in the schists of Aber Hirnant E. of Bala, N.Wales ; 

 limestone and coarse schists of Golden Grove, Llandeilo ; schists 

 half a mile N. of Conway Falls; common in the limestone of 

 Llandeilo, Caermarthenshire ; limestone of Mathyrafal fridd, 

 Meifod, Montgomeryshire, 



{Col, University of Cambridge.) 



Orthisina Scotica (M^Coy). 



Sp. Char. Subquadrate, valves evenly convex, much depressed ; 

 hinge-line as wide as the shell, apical angle 120°, sides sub- 

 parallel, front with an obsolete emargination in the middle; 

 entering valve gently convex, with an obscure narrow mesial 

 depression from the beak to the margin ; receiving valve with 

 projecting beak, not incurved, profile consequently nearly 

 straight, very obscurely subcarinate along the middle, sides 

 gradually sloping to the margins ; cardinal area very narrow, 

 parallel-sided in the entering valve, wide, flat, triangular, 

 and inclining backwards at only 120° in the receiving valve ; 

 pseudodeltidium and deltidial pit concave, foramen oval, near 

 the apex ; surface radiated with twenty-eight thick, simple. 



