288 
DEVONIAN FAUNA. 
Size. — A specimen in Mr. Vicary's Collection measures 15 mm. higli and 
13 mm. wide. 
Localities. — From Lummaton there is a specimen in the Torquay Museum, and 
another, which is very poor, in the Bristol Museum. From Wolborough there is 
a specimen in the Museum of Practical Geology. From Ohudleigh there are five 
specimens in Mr. Vicary's Collection. 
Remarks. — 'This seems a very variable species, so much so that I should have 
been inclined to separate its extreme varieties were they not connected by inter- 
mediate specimens. The variation is caused by the difi*erences in strength of the 
difierent lines of ornament; thus the upper spiral ridges are sometimes large, and 
sometimes very small, and the cross strise on the sinus-band are sometimes strong, 
straight, distant rungs, and sometimes fine, close, arched threads. The shape of 
the shell also varies considerably : sometimes, for instance, the whorls are much 
more overhanging, and sometimes the base is comparatively flat. Nevertheless, 
the specimens all seem to preserve a general character, and I have no doubt that 
they all belong to one species. 
The fossil described by Phillips as PI. monilifera in the ' Pal. Foss.' evidently 
represents this shell. His figure is very defective, and seems to present some 
differences, but his description leaves no doubt of its identity, especially in view 
of the great variability of the species. PI. monilifera, however, as previously 
described by him from Yorkshire/ is totally distinct, and could by no possibility 
be the same as our Devonshire shell. As seen in specimens in the British 
Museum, one of which is figured in the " Geology of Yorkshire," its ornamentation is 
quite different, there being, for instance, no smooth median band. As reproduced 
by de Koninck^ (who gives a long synonomy) it differs in being more turbiniform, 
and in having finer, though somewhat similar, sculpture, no smooth median band, 
a greater sutural angle, and a more circular mouth. 
Affinities. — Troclms Bonei, Steininger,^ with which Phillips compared his 
Devonian species, was figured without a description, but the figure represents a 
shell ornamented with tubercles in a very diff'erent style. 
The Carboniferous PI. Goepjpertii, Goldfuss,* is very similar in shape, but its 
ornament consists of first a plain surface, then a double row of tubercles, then a 
concave sinus-band, and then another double row of tubercles. 
PI. semimida appears to be less trochiform in shape, and to have much coarser 
ornamentation. 
^ 1836, Phillips, ' Geol. Torks.,' pt. 2, p. 227, pi. xv, fig. 10 a. 
2 1842-4, de Koninck, ' Desc. Anim. Foss.,' p. .387, pi. xxxiv, figs. 2 a, 2b. 
3 1834, ' Mem. Soc. Geol. Fr.,' vol. i, p. 371, pi. xxiii, fig. 4. 
* 1844, Goldf., ' Petref. Germ.,' vol. iii, p. 69, pi. clxxxv, fig. 7. 
