RHYNCHONELLA. 131 
1865. RuyNCHONELLA IMPLEXA, Davidson. Brit. Foss. Brach., vol. ili, pt. 6, p. 
67, pl. xiv, figs. 7—10. 
1871. — PARALLELEPIPEDA, var. PENTAGONA, ‘‘ Goldfuss,’ Kayser. 
Zeitsch. Deutsch. Geol. Gesell., 
vol. xxiii, p. 507, pl. ix, fig. 4. 
” 
PLS yL: — TETRATOMA, Kayser. Ibid., p. 526. 
1881. — Inercensis, Kayser. Zeitsch. Deutsch. Geol. Gesell., vol. 
XXxill, p. 382, pl. xix, figs. 2—3 a. 
? 1885. — TETRATOMA, Maurer. Abhandl. Grossh. Hessisch. Geol. 
Landes., vol. i, pt. 2, p. 195, pl. 
vii, figs. 18, 18 a. 
1885. _ PENTAGONA, Maurer, Ibid., p. 196, pl. viii, figs. 19—19 b. 
Localities. —This shell is less common at Lummaton than Fh. parallelepipeda. 
There are thirty-five specimens in my Collection, eighteen in the Woodwardian 
Museum, two in the Bristol Museum, and three in the Torquay Museum. From 
Wolborough there are five specimens in Mr. Vicary’s Collection, five in the 
Museum of Practical Geology, and two in the British Museum. 
Remarks.—A comparison of my specimens inclines me to the belief that 
this shell is distinct from Rh. parallelepipeda. It has no fold on the dorsal valve, 
and only in younger specimens a slight sinus on the ventral valve; the contour of 
the dorsal valve is slightly convex instead of slightly sigmoidal; the margins of 
the valves are less abruptly flattened and more definitely toothed; the plaits are 
fewer, sharper, higher, and extending to the beaks; and the beaks are smaller and 
less elevated. ‘The arrangement of the plaits is remarkable, the four or five 
median plaits which represent the fold being continued to the beak, while the 
lateral plaits are truncated against their sides, so that those nearest to them are 
short, and only the more distant ones reach the beak. The front of the shell is 
slightly concave, and in large shells the dorsal valve has a slight sinus instead 
of a fold. As these differences are constant I think they can only be explained 
on the supposition that we have here a distinct species. It is a smaller shell than 
Bronn’s species, but specimens occasionally reach a fair size, and then the 
difference in the shape becomes still more marked. Hven in smaller shells it is 
sufficiently distinct, though occasionally in this case specimens of rather a 
doubtful nature occur. 
Three or four of my smaller specimens incline me to the belief that Rh. 
pentagona, Goldfuss, as given by Maurer, and possibly also Rh. tetratoma, Schuur, 
sp., and Maurer, are nothing more than small or young forms of the present shell, 
which seems to have sometimes had a slight fold when young. Some of the 
figured specimens of Rh. subcordiformis, Schuur, clearly belong to this species. 
I am strongly of opinion that Rh. Ibergensis, Kayser, is simply an aged form 
or large local variety of this shell. Of the points by which he distinguished it, 
