358 GASTEROPODA OF THE INFERIOR OOLITE. 
row of tubercles. The aperture is subcircular, outer lip lunate and crenulated ; 
columella curved, slightly reflexed and truncated. 
Relations and Distribution.—As a rule, specimens from the Lincolnshire Lime- 
stone are smaller, less elevated, and more distinctly tuberculated than those from 
the Great Oolite of Minchinhampton referred to Monodonta Lyelli. But they vary 
also much amongst themselves. A depressed and more carinated form is 
represented in Pl. XXX, fig. 2, and this tendency is further shown in the var. 
humilis described below. 
It is not always easy, when specimens are indifferently preserved, to distinguish 
between Monodonta Lyelli and Turbo Hamptonensis (Inferior Oolite variety). 
Athough for the sake of reference it is convenient to place these in separate 
genera, so as to change the usual names as little as possible, yet practically they 
belong to the same genus. M. Lyelli may generally be distinguished by the more 
gaping suture, less regular form, more carinated body-whorl, and by the more 
irregular character of the ornaments. It seems to be chiefly confined to the upper 
beds of the Lincolnshire Limestone, notably at Ponton, Barnack, and Weldon. 
I think, too, that it may be recognised at Hook Norton. 
Var. hwmilis, Pl. XXX, fig. 3. More depressed than the preceding ; the height 
averages 4 mm., and the width 5 mm. There are four whorls, the apical ones 
very depressed ; the third whorl is angular and subbicarinate, having two spirals, 
of which the upper one only is tuberculated. The body-whorl, more than half the 
entire height of the shell, is large, bicarinate, and furnished with two or three 
spirals, of which the lower one is often plain; the spirals in the base are finer, 
except the last, which encircles the umbilicus with a richly tuberculated demilune. 
Since the number of whorls is the same, this form cannot be regarded as the 
young of M. Lyelli, but rather as a stunted and local variety, which is itself very 
variable, and only partly represented in fig. 3 of the accompanying plate. Most of 
my specimens are from Weldon. 
293. TurBo ‘‘ DEPRESSIUSCULA.” Plate XXX, fig. 4. 
Description: 
Height : : : . 9 mm. 
Width , é 5 (8) Wahaal, 
Shell obtusely conical, subdepressed, umbilicate ; apex obtuse, consisting of 
two smooth and flattened whorls; third whorl bicarinate with two richly 
tuberculated spirals, the tubercles being connected by short axial coste; sutures 
wide and canaliculate. The body-whorl is angular and strongly bicarinate, having 
