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 PI. 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. humilis, PI. 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 . . . . .5 mm. 



Width . . . . .8 mm. 



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 costas ; sutures 



wide and canaliculate. The body- whorl is angular and strongly bicarinate, having 



