CAMBRO-SILURIAN FOSSILS. 221 



the striae are thicker and coarser; internally the difference is 

 still greater, the present species having much shorter and wider 

 muscular impressions in the receiving valve, and the charac- 

 teristic broad arrow-shaped impressions of the rostral portion of 

 the entering valve, produced by the comparatively long diver- 

 ging dental lamellae, wholly separated from the mesial furrow. 

 The gibbous entering valve with its deep narrow mesial sulcus 

 near the beak, coarser striae, and totally different internal cha- 

 racters separate if from the O. retrorsistria (M'CJoy), with which 

 it also frequently occurs. 



Extremely abundant in the decomposing sandy schists of Aber 

 Hirnant E. of Bala, N. Wales. 



[Col. University of Cambridge.) 



Orthis turgida (M'Coy). 



Sp. Char. Longitudinally ovate, globose, margins very obtuse, 

 greatest width slightly in front of the hinge-line, which is 

 slightly less than the width of the shell ; receiving valve very 

 gibbous, profile much arched by the large beak, declining 

 nearly to the level of the lateral margins, greatest depth about 

 the middle of the length ; in old specimens there is a faint 

 wide shallow depression towards the front margin, which gra- 

 dually changes into an obscure prominence towards the beak, 

 or in the young ; sides tumid ; beak long, apical angle 100°, 

 entering valve extremely gibbous, sometimes hemispherical, in 

 some specimens more depressed, but remaining remarkably 

 tumid towards the margins ; a deep narrow sulcus extends from 

 the beak almost halfway to the margin, becoming rapidly effaced 

 by widening and flattening towards the front, which is de- 

 pressed, with a narrow sinus in young specimens (from the 

 mesial sulcus), and very slightly raised with a wide shallow 

 wave (not affecting the surface) in old individuals (from the 

 slight mesial depression of the old receiving valve) ; both 

 valves covered with numerous fine, sharply defined, obtuse 

 striae, separated by flattened spaces, equalling them in width, 

 bifurcating two or three times between the beak and margin, 

 where, in old individuals, they are a little finer than in the 

 middle of the shell, eight or ten in two lines at six lines from 

 the beak, about ten to twelve at the front margin at ten lines 

 from the beak ; cardinal area in receiving valve very large, flat, 

 nearly half as high as wide, inclining backwards at 130° ; 

 foramen narrow, triangular, entirely open ; area of entering 

 valve flat, triangular, about seven times wider than high : 

 internal cast of receiving valve with strong dental lamellae, 

 diverging at 50°, forming the posterior lateral boundaries of 

 a narrow ovate, strongly defined pair of muscular impressions. 



