44 FAUNA OF THE CORNBRASH. 



Description. — The specimen which most nearly resembles the type is figured in 

 PL III, fig. 6. Its numerical characters are given in No. 26 of Table I (p. 40). 

 As in all the specimens referred to this species, the thickness is great and the ribs 

 are few. The umbilicus is small and its walls are vertical within. The ribs are 

 sharp and, as near as may be, radial, and increase in number almost entirely by 

 bifurcation near the umbilicus, and the divisions, except at the bifurcation, where 

 the elevations are combined, are nearly equal throughout. The greatest thickness 

 is not at the umbilical edge but some distance away, and thence there is a funnel- 

 like depression leading down to the ultimately vertical ribbed edge. 



The smallest specimen referred to this species (M. P. G\, 8653) has a diameter 

 of 25 mm. (see No. 2 in Table I, and PI. Ill, fig. 4). The last whorl is here so 

 very convex laterally, that though it leaves one fourth of the penultimate whorl 

 exposed, it nevertheless overhangs part of the latter ; the united ribs at each 

 junction, where is the greatest breadth of the shell, are so much raised as almost to 

 look spinous. The inner slope gives a funnel-shaped appearance to the umbilicus. 

 The riblets take on their full size as soon as possible ; they are mostly grouped in 

 threes, two of which join, and the joined ones pass over the umbilical slope. 

 Comparing one side of the shell with the other, any pair of peripheral ribs which 

 unite to form one umbilical rib on one side pass to consecutive umbilical ribs on 

 the other — i. e. the ribs mismatch. Another fragmentary specimen of about the 

 same size, from Appleby, in Lincolnshire, shows similar characters, but the riblets 

 match on the two sides, as they do also on the inner whorl, where the diameter 

 is only 7 mm. 



The next stage is seen, No. 14 of Table I (PI. Ill, fig. 5). In this stage it 

 closely resembles the figure given by Reinecke (fig. 47) of N. tumidus. The 

 greatest convexity is found near the inner third of the outer whorl, but the whole 

 surface is uniformly elliptical in a transverse— i. e. radial — direction, the pseudo- 

 spines forming the highest line. This convexity gives the apparent umbilicus 

 a wider opening. 



The final stage, after the typical adult form has been passed, is a gradual 

 failing of the ribs till the surface is smooth. At 10 in. diameter and a thickness 

 of 7 in. only a few ribs are seen at the smallest part of the whorl. This is a 

 common feature whenever the shells are large, as seen also in M. compressus from 

 Garsdon, but in the present case it has induced the introduction of the name 

 terebratus given by Phillips to a shell which he describes as more globular than 

 A. hervei/i, and which, therefore, he intends for the present species. Representa- 

 tives of this senile form occur in Scarborough and York Museums. They are 

 terebrate M. macrocephalus. 



Distribution. — Specimens definitely determined as belonging to this species have 

 been seen from Scarborough (9), Appleby (1), Rushden (=3), Aldwinkle (1), Castor 

 (1), Fineshade (1), Northants (1), Peterborough (1), and Bedford (1). It is 



