496 W. J. SOLLAS ON THE SILURIAN DISTRICT 
Descriptions of and Notes on Fossils. 
1. Moproxorsis AcurrIpRoraA, sp. noy. (PI. XXIV. figs. 21 & 22.) 
Shell ovate, convex transversely; umbones not prominent, dia- 
gonal ridge faint, existing only as a line of division between two 
portions of the shell—one larger, antero-inferior, and with a gentle 
convex slope downwards, the other smaller, postero-superior, and 
with a somewhat steeper, flatter slope towards the hinge-line. No 
diagonal furrow. Inferior margin a simple elliptical outline. Pre- 
umbonal lobe somewhat ovate, or like a blunted lancet-point in 
shape. Angle formed by the junction of the hinge-line with the 
posterior margin obtuse and rounded. Surface covered with very 
fine concentric strice. 
Length 148"; breadth 18”. 
Remarks. This shell very closely resembles the Upper Cambrian 
species MM. expansa (Portlock), but differs from it in having a more 
curved ventral margin, a sharper preumbonal lobe, a relatively larger 
antero-inferior and smaller postero-superior region, and in being less 
compressed. 
Casts of the interior and exterior of the shell are exhibited in the 
same specimen; the internal cast shows a large kidney-shaped im- 
pression of the anterior adductor muscle, situated inside the per- 
umbonal lobe, a very distinct simple pallial line parallel with the 
inferior margin, and a deep round impression of the anterior pedal 
muscle, just above that of the anterior adductor. The ligamental 
groove is just visible along the hinge-line. The space between the 
outer and internal casts is very narrow; so that the shell must have 
been exceedingly thin. 
2. Mopiotopsis inruata, M‘Coy. (Pl. XXIV. fig. 2.) 
The determination of this species rests on a well-preserved in- 
ternal cast, which presents all the characters of M‘Coy’s species, 
the only difference noticeable lying in the somewhat greater elevation 
of the Cardiff form, and the slightly greater incurving of the ventral 
margin immediately in front of the diagonal gibbosity. The di- 
mensions of the Cardiff form are—length 0°85", breadth 0-525", 
giving a proportion of 134%), while in M: Coy’s form the proportion 
is 1779 Perhaps my sumer should be regarded as a variety of 
570 
M., inflata, i in which case it might be called Mu. elevata. 
3. ORTHONOTUS NAVICULA, sp. nov. (Pl. XXIV. figs. 3, 3a.) 
Shell elongate, oblong, tumid, especially above; beaks small, 
meeting over the straight hinge-line in the anterior part of its 
middle third ; lunule shallow, well defined. A transverse furrow in 
each valye, commencing from the point of the umbones, and de- 
scending vertically to the ventral margin, where the two furrows 
meet. Immediately in front of the cincture so formed the shell 
bulges out rapidly ; behind the cincture the bulging takes place more 
gradually. 
An oblique ridge extends from the posterior side of the beaks to 
