﻿ORTHIS. 



93 



Ortiiis arcuata, Phillips. PI. XVII, figs. 13, 14. 



Orthis arcuata, Phillips. Pal. Foss. of Cornwall, Devon, and West Somerset, p. 64, 

 pi. xxvi, fig. 107, 1811. 

 — — WCoy. British Pal. Foss., p. 384, 1852. 

 ? — longi sulcata, Phil. Pal. Foss., p. 64, pi. xxvi, fig. 105 (?), 1841. 



Spec. Char. Depressed, transversely elliptical, wider than long ; hinge-line much less 

 than the width of the shell. Dorsal valve gently convex, ventral one very slightly convex 

 or nearly flat; areas narrow. Surface of both valves marked by numerous radiating striae, 

 or minute, thread-like ribs, which commence to bifurcate at variable distances from the 

 beaks. 



Dimensions. Two specimens measured — 

 Length 7, width 11, depth 2 lines. 

 » 6 ,, 8 „ 3 „ 



Obs. It is not an easy matter to obtain complete and satisfactory examples of this 

 species ; the greater number of specimens obtainable from the Middle Devonian beds of 

 Hope's Nose, near Torquay, being much out of shape from the effects of pressure; nor is 

 the dorsal valve always flat and hollowed along the middle, as is described and figured in 

 one of Phillips's specimens. 



I am not quite certain that I am justified in considering 0. longisulcata (Phillips) 

 as a synonym of 0. arcuata, but must observe that the last-named author's 0. longisulcata 

 was described from casts and impressions so entirely out of shape from the effects of 

 pressure, that it was not possible to ascertain the shell's true character. What has tempted 

 me to provisionally locate 0. longisulcata with 0. arcuata is, that many examples of 

 0. arcuata from Hope's Nose entirely resembled in shape some of Phillips's specimens 

 of 0. longisulcata ; we will, however, here append Phillips's description, and I have repro- 

 duced one of that author's illustrations (fig. 17 of our Plate). 



" Orthis longisulcata, Phillips, Pal. Poss., p. 62, pi. 26, fig. 105. Character. Trans- 

 versely elliptical, depressed, radiatingly striated, with numerous imbrications ; striae very 

 numerous, fine, devaricating ; internal plates near the beak of the lower valve, long, 

 diverging. If my views be correct, the sides of this species are rounded so as to make 

 the whole contour elliptical, as in 0. lata (Sil. Researches). The long, straight, sub- 

 rostral plates appear to be constant. I have seen innumerable specimens at Watersmeet, 

 Linton, and Woodabay, but almost always obliquely crushed. The striae are much finer 

 than in the elliptical species from Petherwin (0. interlineata), to be noticed next. 

 Loc. In North Devon. Linton, Watersmeet, Woodabay, West Lee." 



