304. MM. Wilson and Crick—The Lias Marlstone of Tilton. 
rotulus, Stol., must not be confounded with Trochus ta 
Moore, from which it 1s quite distinct. 
Marlstone Rock, Tilton (Hast Norton embankment). 
PLEUROTOMARIA HELICINOIDES, Roemer, 1836. Plate IX. Figs. 13a, 130. 
1836. Trochus helicinoides, Roemer, Die Verstein. des Ool.-Gebirges, p. 150. 
pl. x1. f. 13. 
Syn. 1867. TZrochus carinatus, Moore, ‘“ Middle and Upper Lias of the South- 
West of England,’ Proc. Somerset Arch. and 
Nat. Hist. Soc. vol. xiii p. 207, pl. 4, fs. 24, 25, 
non Turbo canalis, Minster, nec Plewrotomaria helicinatdes. Roemer, 
of Tate. 
Whilst agreeing with Mr. Ralph Tate, F.G.S.,* that Trochus 
carinatus, Moore, is a Pleurotomaria, and (in all probability) identical 
with Trochus (not Turbo) helicinoides, Roemer, I cannot go so far as 
to admit that these are the same as Turbo canalis, Minster—a 
Pleurotomaria truly, but a different species I maintain, to the above. 
The figure entitled Pleurotomaria helicinoides in the Yorkshire Lias 
is, I consider, an illustration of ‘ Zurbo’ canalis, Mi., and not of 
‘Trochus’ helicinoides, Roemer. 
The following descriptions and the accompanying figures of Tilton 
specimens will indicate the chief points of difference between these 
two forms. 
Pleurotomaria helicinoides, Roemer (assuming this to be the 
equivalent of Trochus carinatus, Moore), is a smooth and even polished 
shell, with very clean cut and angular sculpturing; the whorls have 
an acute keel anterior to the middle line; on this keel is placed the 
sinus-band, which is bounded by a single rather widely-spaced 
raised line on each side; from the sinus-band the whorl! falls verti- 
cally in front to the anterior suture, and slopes gently back in a 
single concave sweep to a raised line or faint keel close to the 
posterior suture; the last whorl bears a third angulated keel ante- 
riorly, bounding the broad vertical area below (i.e. anteriorly) ; 
the shell is covered with very fine curved lines of growth; the 
base is only slightly convex, smooth, but bearing a few very fine 
acute concentric lines either limited to the outer part or continuous 
to the centre; there is a very small umbilicus; the aperture is 
transversely ovate, a ill-defined columella. Height, 8 mm. ; 
ereatest diameter T mm.; Spiral angle convex, about 70°. 
Marlstone Rock, Tilton (East Norton embankment). 
Prievrotomarta (TurBo) cAnatts, Minster, 1848. Plate IX. Fig. 14. 
1848. Turbo canalis, Pe. gous, Petref. Germ. vol. ii. p. 95, pl. 198, 
figs. 12a, 0. 
Syn. 1878. Plewrotomaria helicinoides, Roem. sp. Tate, non Roemer, ‘‘ The York- 
1 shire Lias,’’ p. 338, pl. x. figs. 7a, 70. 
Whilst possessing the same general form of Pl. helicinoides, as 
above described, this shell presents rounded instead of angular 
contours, is far from smooth, and differs in its proportions as well 
as in its ornamentation. In Pl. canalis the keel bearing the sinus- 
band is situated posteriorly rather than anteriorly to the middle line, 
1 “The Yorkshire Lias,’’ by Tate & Blake, p, 338, pl. x. fs. 7, 7a. 
