374 GASTEROPODA OF THE INFERIOR OOLITE. 
more than half the total height. Number of whorls seven; sides concave, the 
posterior and anterior margins decorated by nodular carine. A wide and regular 
suture separates the lower carina of one whorl] from the upper carina of the next ; 
the intercarinal spaces have no spiral ornamentation, but very fine axial lines may 
be seen on the glabrous surface. 
The body-whorl is similar, but with a double nodulated carina round the base, 
which is flat and smooth, but rises towards the centre, where an umbilicus of 
moderate width and depth is girdled by a set of large tubercles, about eight in 
number. Aperture subrhomboidal and depressed. 
Relations and Distribution.—This species is, perhaps, the most abundant 
Gasteropod in the Upper Division of the Inferior Oolite, being especially charac- 
teristic of the Parkinsoni-zone from Burton Bradstock as far north at least as 
Aston in the Cotteswolds. I have no specimens either from the Lincolnshire 
Limestone or from Yorkshire. It is essentially a Bajocian (i. e. Upper Division) 
form, being abundant at Bayeux, &c. French specimens seem to be rather 
wider-angled than ours. There is a marked variety from Powerstock in Dorset, 
which I have not figured. 
This is very different to the polymorphous species usually known as Trochus 
subduplicatus, d’Orb., from the Lower Beds, but is closely related to the two forms 
next described. 
311. Troonus aneutatus, Sowerby, 1817. Plate XXXI, fig. 11. 
1817. Trocuus concavus, Sowerby. Min. Conch., pl. elxxxi, fig. 3; op. cit., 
vol. iv, index and corrigenda, 1823, as 
TROCHUS ANGULATUS. 
1854. — ANG@ULATUS, Sow. Morris, Cat., p. 281. 
This form is somewhat wider than average specimens of 7’. duplicatus, of which it 
may, to a certain extent, be considered a glabrous variety. Sowerby relied upon 
the presence of a few transverse striz as helping to separate it. Though this test 
fails, the following important differences may be noted, viz. the extreme smooth- 
ness of the shell, the absence of umbilicus, and the fusion of the two keels 
into one. 
Intermediate forms which show the connection, but which incline more towards 
T. duplicatus, occur in several places. Specimens, such as the one figured, 
are rare. 
