﻿ACTINOPTERIA. 



65 



seventy distant, elevated, rounded, and usually alternating rays, which are sepa- 

 rated by flat interspaces, generally wider than the rays ; the whole crossed by 

 occasional indistinct bulges, by minute, close, regular, rounded, transverse strise, 

 and in some well-preserved specimens by rather regular, distant, fine, foliaceous 

 threads. Shell-structure thin. 



Right valve large, flattish, oblique, slightly transverse. Umbo apparently 

 prominent, oblique, anterior. Anterior margin receding. Inferior margin 

 obliquely convex. Infero-posterior corner broadly convex. Posterior margin 

 straight, erect. Posterior wing large, broad, triangular, undefined, extending the 

 whole length of the shell. Contour of surface convex near the umbo, flattened 

 towards the margins. Surface with about twelve low, rounded, small, distant 

 rays on the posterior wing, and about ten, which are still less distinct, on the 

 adjacent parts of the median region ; elsewhere without rays ; the whole shell 

 being covered by fifty or sixty sharp, minute, and very distant, transverse ridges, 

 the steep sides of which face the umbo. 



Size. — A left valve measures about 45 mm. long, 36 mm. broad, and 12 mm. 

 deep. A right valve apparently measures about 60 mm. long, 47 mm. broad, 

 and 10 mm. deep. 



Localities. — There are fifteen examples of the left valve in my Collection from 

 Lummaton, and four of the left valve and one of the right valve in Mr. Vicary's 

 Collection from Wolborough. 



Remark*. — The left valve of this species seems characterised by its convexity, 

 its transverse sub-quadrate shape, its low, rounded, distant ribs, and its small, 

 regular, close striee. 



It seems to vary considerably both in length and in shape, as well as in the 

 number of ribs, the latter quality being apparently due to the smaller of the 

 alternating ribs being in some cases obsolete. Some of the extreme forms, 

 perhaps, may ultimately prove to be distinct ; but, as it does not seem possible to 

 draw any definite line between them, I have thought it best to leave them together. 

 Thus several specimens show a narrower and more oblique anterior side, and 

 in these the distant transverse threadings are more clearly seen. Hence they 

 approach A. intermedia, CEhlert, 1 though still being much less triangular in shape 

 than that species. But to prove that these small differences are more than 

 accidental would require a much more perfect series of specimens than are at 

 present at command. 



The largest specimen from Wolborough (PI. VI, fig. 8) seems to be much flatter 

 than the other shells, and its rays are more numerous and less prominent. I am 

 inclined to think that these characters are to be accounted for by the fact that it 



1 1881, CEhlert, 'Mem. Soc. Geo). Fr.,' ser. 3, vol. ii, p. 21, t. 3, figs. 1— lc. 

 VOL. II. 9 



