438 GASTEROPODA OF THE INFERIOR OOLITE. 



Shell conical, turrited, only slightly umbilicate. Spire regular and sharp, 

 except just at the apex, where it is rather flattened. Whorls in a medium-sized 

 specimen seven, angular, the posterior third having a moderate slope, and the 

 anterior two-thirds being nearly vertical ; sutures close and not canaliculate. The 

 keel of each whorl carries a coronet of undulating tubercles, and the whole shell is 

 pervaded by fine and close undulating spiral ornament, which is decussated by 

 extremely fine radial lines. 



The sinus-band is wide, flat, and submedian, with spiral striae decussated by 

 growth-lines, and this kind of ornamentation is continued throughout, except in 

 the body-whorl of very large specimens. The body-whorl is large and biangular; 

 the lower keel, or basal periphery, is scarcely tuberculate, and in very large 

 specimens almost smooth. The base is flat-convex with extremely fine reticulate 

 ornament ; the umbilicus is small and inconspicuous, but rather deep in some of 

 the younger forms. Aperture square to subcircular. 



Relations and Distribution. — In PI. intermedia, Minister (Goldfuss, pi. clxxxi, 

 5), which is said to occur in the Lias, we have in many respects a very similar 

 shell ; but in that species the sinus-band is represented as occurring on the 

 keel. PL subaraneosa is little more than a modified form of PI. araneosa, var. 

 reticulata, Deslougch. (vol. cit., p. 89, pi. xiv, fig. 5), thus affording another 

 instance of a Liassic survival. Our shell, however, is rather more subdued in 

 ornaments and presents some minor points of difference ; which facts, coupled 

 with its comparative abundance in the Concavus-beA at Bradford Abbas, seem to 

 entitle it, provisionally at least, to be regarded as a distinct species. 



At Bradford Abbas the majority of the specimens are immature, consisting of 

 about five or six whorls. The fine and close reticulate ornamentation is well 

 exhibited in some of these, and there is a specimen from the same horizon at 

 Beaminster where the close and delicate spiral system in the base is well brought 

 out. On the other hand, the base of the larger specimens appears smooth (? from 

 wear). Individuals with eight whorls attain to a large size (basal diam. 80 mm.). 

 In these the umbilicus is almost closed, and the shells present the appearance of 

 having been worn. There is a specimen from Dundry, in Mr. Wilson's Collection, 

 where the ornamentation in the spire is bolder (or better preserved), and this still 

 more nearly resembles PI. araneosa. 



In this connection, also, I would draw attention to a specimen (PI. XXXIX, 

 figs. 2 a, b) from the Liassic Sands of Gloucestershire, where the ornamentation is 

 bolder than in PI. subaraneosa, and altogether more like that of Deslongchamps' 

 species. N.B. — The actual specimen does not warrant the amount of umbilicus 

 shown in the figure. 



Secondly, there remains for consideration the large species of Pleurotomaria 

 occasionally found in the lower part of the Yorkshire Dogger, and identified by me 



