ORTHIS. 129 



Var. D.— S. radialis, Phillips. PI. XXV, figs. 10— IS. 



SPIRIFEB uadiams, Phillips. Geo], of Yorks., vol. ii, pi. xi, lig. .'), 1836; and l)av., 



Mon. of Scottish Garb. ISracliiopoda, pi. i, figs. '21, 

 25, 18G0. 



Shell variable in shape ; transversely or longitudinally semicircular ; hinge-line usually 

 shorter than the greatest width of the shell. Dorsal area of moderate width ; fissure closed 

 by a pseudo-dcltidium. Valves moderately convex, the dorsal one being the deepest, with 

 sometimes a slight depression near the front of the ventral one. Surface covered with strong 

 radiating striae, with intervening smaller ones, crossed by imbricated lamellae and deep 

 concentric lines or interruptions of growth. 



This species or variety (?) does not appear to have attained the large dimensions of S 

 crenistria, but the following average measurements may be given : 

 Length 17, width 21, depth 7 lines. 

 >> lu, ,, Id, ,, () ,, 



The interior details do not appear to differ from those of S. crenistria ; but on a young 

 specimen from Gare there appeared to exist a small circular foramen at the extremity of 

 the beak, similar to what we see in some species of Strophomena ; the aperture does not, 

 however, exist in full-grown individuals, and thus shows that the animal could dispense 

 with its temporary moorage. 



The striae differ also much in different specimens, from the larger ones being either closer 

 or more widely separated, two or four smaller ones occupying the interspaces. It must 

 be, however, observed that it is difficult at times to distinguish certain individuals of 8. 

 radialis from others of crenistria. 



Professor Phillips mentions Florence Court, near Enniskillen, as the locality whence 

 his type was obtained ; but the figure in the ' Geology of Yorkshire' is very incomplete, and 

 drawn from a crushed specimen. Good examples have been collected at What ley, near 

 Frome, in Somerset; at Gare and Middleholm, in Lanarkshire, Scotland ; and at Middleton, 

 in Ireland. 



Genus— ORTHIS. 



The genus Orthis forms a well-characterised group, especially numerous and abundant 

 in the Silurian and Devonian systems, is considerably reduced during the Carboniferous 

 period, to appear no longer (?) in subsequent stages. Four species alone have been 

 hitherto discovered in the Carboniferous rocks of Great Britain. 



17 



