198 E. Wilson — British Liassic Gasteropoda. 



deeply cut but very narrow suture ; these are crossed by numerous 

 slender, slightly oblique, radial costse, forming with the spiral lines 

 a neat meshwork, having little nodules at the decussations ; the 

 radial costae are more slender and more closely set on the last 

 whorl. Base almost flat, bearing numerous fine encircling threads 

 with nearly equal interspaces; the radial costge are continued over 

 the angular border of the last whorl, to which they give a fine 

 crenation, and then rapidly die away. Aperture depressed, lunate 

 and oblique, outer lip somewhat thickened, columella inconspicuous. 



Dimensions. — Length (restored), 6-75 millimetres; diameter, Imm.; 

 spiral angle, 35°; sutural angle, 110°. 



Affinities. — This form appears to be related to Trochus Holwelhnsis, 

 Moore, a fossil derived from the Liassic deposit contained in fissures 

 of the Mountain Limestone near Frome (QJ.Gr.S. vol. xxiii. p. 554, 

 pi. 17, figs. 1, 2). The types of this species should be in the Bath 

 Museum, but they are unfortunately missing. Comparison of our 

 specimen with Moore's figure, however, shows that whilst the two 

 fossils agree in general form and ornamentation, Tr. Holioelhnsis 

 had a more elongated spire, the whorls of which, instead of im- 

 bricating each other, were separated by wide and deep sutures, and 

 bore fewer and coarser radial costse, making prominent " bosses " on 

 the margin of the last whorl. Moore's type appears also to have had 

 a small umbilicus. These points of difference seem quite sufficient 

 to distinguish the two forms specifically. 



Geological Position and Locality. — Upper Lias (transition-bed to 

 Middle Lias), L. and N. W. Eailway, Watford, Northamptonshire. 



Tkochus Northamptonensis, spec. nov. PL V. Figs. 5, 5a. 



Description. — Shell conical, imperforate ; apex acute ; spiral angle, 

 regular or slightly convex ; whorls 7, concave ; a prominent and 

 acute crenulated ridge or carina encircles the anterior portion of the 

 whorls, and a similar but much less prominent ridge encircles the 

 whorls close to the posterior suture ; between these there is a 

 concave area occupying two-thirds of the width of the whorl ; from 

 the anterior carina the surface of the whorl falls vertically to the 

 anterior suture, this portion occupying about a third of its width ; 

 the last whorl is tricarinated, having two anterior crenulated carinas, 

 the posterior of which is the more prominent ; broad undulating 

 costae run somewhat obliquely from the spinose points of the 

 posterior to those of the anterior carina, beyond which they are 

 continued vertically to the anterior suture, and on the last whorl, 

 after connecting the spinose points of the two anterior carinee, 

 are continued in more or less prominent serpentine ridges from the 

 circumference to the centre of the base. A few faint encircling lines 

 are discernible in the concave portions of the whorls. The whole 

 shell is covered with fine close-set radiating flexuous lines, which pass 

 over the costae and their interspaces, the carinas and the base. The 

 base is only slightly convex, but prominent in the centre, and marked 

 with numerous concentric lines, which are either equally distinct and 



