RHYNCHONELLID®. 175 
RHYNCHONELLA DIODONTA, D’Lichwald. Lethzea Rossica, p. 759, 1859. 
—_— BOREALIS, Salter. Siluria, 2nd ed., p. 544, pl. xxii, fig. 4, var. 
fig.5, 1859. 
— —_— var. proponta, Id. Ib., pl. xxii, fig. 5. 
— _— Lindstrim. Proceed. Royal Acad. of Stockholm, p. 365, 
1860. 
Spec. Char. Subtrigonal, wider than long. Valves more or less gibbous, dorsal valve 
always most gibbous. Beak of ventral valve moderately produced and incurved, foramen 
clearly exposed, and situated beneath the angular extremity of the beak; it extends to the 
hinge-line, and is margined on either side by narrow deltidial plates; sinus broad, deep, 
and extending from the extremity of the beak to the front. In the dorsal valve the fold is 
abruptly raised, with wide smooth walls, but flattened along the middle, and varying in 
breadth in different specimens. Surface of valves ornamented with from ten to twenty 
angular ribs, of which from two to six compose the fold, while from one to five occupy the 
sinus. ‘The ribs are usually simple, but sometimes, as an exception, one or two dichoto- 
mise, the whole being crossed by fine transverse striz. On either side of the beak, between 
the beak-ridges and hinge-line, a concave smooth space occurs, while the lateral spaces on 
either side of the beak are moderately wide; the margin of the ventral valve indenting 
by a convex curve the corresponding edge of the dorsal valve. Interior not known. Three 
specimens measured— 
Length 14, width 17, depth 12 lines. 
ees) a ee Os oy 
es a, 8 gee 
Obs. ‘This species varies greatly in shape, according to age and individual, and 
especially so from the number of ribs which occur on the fold and sinus ; and this has 
tempted Dalman and others to distinguish some of these varieties by the separate spe- 
cific denominations of diodonta and bidentata. Myr. Salter observes, when describing 
Ith. borealis, that the shell is conspicuous, even in its young state, by the two raised 
dorsal ribs and the deep furrow in the ventral valve with its single rib ; that, if this number 
is retained, it becomes 7. diodonta, Dalman, and that perhaps even 7. didentata, id., 
may be the young of it ; but that with respect to the former, he is quite sure it is onlya 
variety, and that Dalman himself has hinted at the probability of this, the Z! d:dentata of 
the ‘Sil. System’ being a synonym of 7. diodonta, &e. Having assembled a large number 
of specimens of RA. borealis from the Wenlock Limestone of Dudley, where the shell is 
abundant, I found that in some young examples the two ribs of the small variety diodonta 
were also present in large adult individuals of Rh. borealis, such as in figs. 16 and 18, 
while in other specimens there are three, four, and more, on the fold, as in figs. 14, 
15, and 24; but in other young examples of RA. borealis three, four, and even five ribs 
are present on the fold; and these are persistent into the adult state, so that it seems 
