432 GASTEROPODA OF THE INFERIOR OOLITE. 
Description : 
Height : ‘ : 5 . 45 mm. 
Basal diameter : : ; . 75 mm. 
Spiral angle . ; ; ; “, L20°2 
Shell turbinate, depressed, umbilicate. Spire convex. Whorls (six to seven) 
subangular, and increasing by steps with a canaliculate suture. The crest of each 
whorl is surmounted by a diadem of tubercles, rather small and oblique but 
somewhat closely set, whilst flexuous spiral lines ornament the sides of all the 
whorls ; moreover in some cases the posterior margin of each whorl shows a ridge 
lying between two furrows, in the upper of which is the suture, whilst the lower 
furrow constitutes a sort of trough immediately behind the diadem of tubercles. 
In other cases there is a slight anterior keel at the base of each whorl. 
The sinus-band is broad and median, and shows three spirals in the earlier 
whorls ; but there is great variety in this respect, and also in the degree of its 
prominence; in the body-whorl of very large specimens it is flat and scaly. The 
body-whorl is relatively very large and biangular, the upper carina being 
surmounted with tubercles, whilst the lower one, forming the angle or basal 
periphery, is little more than crenulated in some specimens. The base is 
subconvex and without any special ornament, but the flexuous growth-lines are 
very conspicuous, and may be traced into the deep and funnel-shaped umbilicus. 
Aperture subovate-depressed. 
The younger forms have a strongly dentate periphery and actinomphaloid base, 
but the umbilicus is not so wide as in Pl. actinomphala. 
Relations and Distribution —This is one of the representatives, on a higher 
horizon, of Pl. actinomphala; but the spire is more convex, the ornaments are 
different, the body-whorl is more angular, and the rugose character of the base 
appears to be modified. There is, however, much variety. 
It was quoted by d’Orbigny from Dundry, from which locality I have one 
specimen. By far the best specimens come from the Sauzei-bed at Oborne. I 
have one in a good state of preservation which has a basal diameter of 100 mm. ; 
in this one the basal periphery has become quite smooth. No other horizon in 
Dorset has, to my knowledge, yielded PI. Actzxa. 
1 D’Orbigny gives the spiral angle as 102°, but his figured specimen shows an angle of 110°. It 
seems probable that the Dorset specimens are wider on the whole. 
