294 
DEVONIAN FAUNA. 
exposed to about the centre of the back. Sinus-band narrow, flat or concave, 
prominent, situate upon the shoulder just above the widest part, bounded 
and sometimes bisected by fine or coarse threads. Ornament consisting of nine 
to fourteen fine, alternating, spiral threads above the sinus-band, and very nume- 
rous similar alternating threads below it and upon the base. Transverse orna- 
ment consisting of a few more or less indistinct broad bulges near the suture, and 
very fine oblique striee, which are seen to be very acutely deflected at the sinus- 
band, but are so indistinct as to be almost invisible. Mouth unseen. Umbilicus 
small, deep. Shell-structure thin. 
Size. — A specimen measures 15 mm. high and 13 mm. wide; another measures 
20 mm. high and 19 mm. wide. 
Localities. — In the Woodwardian Museum are two specimens from Wol- 
borough, which were described by M'Coy as PL imhricata, F. A. Romer, and a 
third from Lummaton, which was collected by Mr. B. B. Tawney and referred by 
him (on the museum label) to PL aspera, Phillips. In the Museum of Practical 
Geology are three examples from Wolborough, and in the Torquay Museum is a 
small specimen from Lummaton. 
Bemarhs. — The distinctive features in this shell are the narrow, elevated sinus- 
band, and the numerous fine and rather irregular spiral threads on the rest of the 
surface. The transverse ornament is so fine as rarely to be detected. In Mr. 
Tawney' s specimen the spiral threads are more numerous and alternating than in 
the Torquay fossil, and the sinus-band is deeply concave, while in the latter it is 
filled by one or two coarse threads. One of the specimens in the Museum of 
Practical Geology has the sinus-band lower down and the spiral lines above it 
very fine ; it presents rather a peculiar appearance, due to the slightly convex and 
oblique upper parts of the whorl, which, being very wide and enveloping a great 
part of the whorl above, cause the spire to have a gently undulating contour, broken 
only by the flatter perpendicular sinus-band. I formerly regarded this specimen 
as distinct, but a further examination leads me to believe that it is nothing more 
than a variety of the present species. 
PL imhricata, as originally described by F, A. Romer,^ is extremely like the 
English fossils, but it presents some minor differences from them. Thus the 
transverse striae are more strongly marked, the sinus-band is much wider, 
the sutures are deeper, and the shape of the shell is flatter. Clarke" redescribes 
Romer's specimens, and states that the transverse marks are really almost imper- 
ceptible, but he describes the shell as even flatter than does Romer, and the 
sinus-band as being much higher or broader than it is in the English fossils. I 
am therefore in much doubt as to whether M'Coy is to be followed in his identifi- 
1 1843, F. A. Romer, ' Verst. Harzgeb.,' p. 28, pi. 8, fig. 1. 
2 Clarke, ' Neues Jahrb. f. Min.,' Beil.-Band iii, p. 341. 
