152 E. S. Gobbold — Cambrian Hyolithidai from Hartshill, 



each lateral angle; these cause the angles to project something like 

 those of H alatus ; the ventral side is, however, bluntly keeled. 



Horizon and Locality. — Lower Cambrian : red sandy limestone of 

 Woodlands Quarry, Hartshill. 



Hyolithus (H.) biconvexus, sp. nov. (PI. IV, Figs. 10-12.) 



? Hyolithus cf. lenticularis, Linnarsson, Lapwortli, Proc. Geol. Assoc, vol. xv r 

 pt. ix, pp. 343, 349, 1898. 



Diagnosis. — Shell straight, taper 1 in 3 to 1 in 4 (equivalent 

 apical angle 18° to 14°), length of type-specimen 14 millimetres, 

 diameter of mouth in another specimen 5 mm. Section biconvex, 

 dorsal face moderately convex, ventral strongly convex, ratio of 

 diameters 1 to '66, lateral angles rather sharply rounded; aperture 

 in .two planes, dorsal lip projecting but little; surface marked by 

 fairly strong ridges of growth, spaced about eight to the millimetre in 

 the body of the shell, but closer near the aperture, and having 

 a tendency' to be alternately strong and weak; the ridges are 

 strongly bent forwards on passing the lateral angles; interior, 

 surface apparently smooth. 



Operculum (?) : one or two examples of opercula that may belong 

 to this species have been found. They are constructed on the same 

 general lines as those assigned to H. alatus, but are more oval in 

 outline as usually seen in the rock, the ventral margin does not 

 rise to so great a height above the convex conical portion of 

 the plate, and the sides of the conical portion meet at an angle of 

 about 90°. 



Comparison tvith other species. — H. biconvexus has a similar section 

 to that of H. acadica (Hart MS.), Walcott, 1 which, however, has faint 

 longitudinal strise and a greater apical angle. 



The Middle Cambrian form H. socialis, Linnarsson, 2 has a some- 

 what similar section, but a smaller apical angle, and is provided with 

 a rounded keel on the ventral side. 



H lenticularis, as figured by Holm, 3 has its dorsal and ventral 

 sides equally convex. 



Horizon and Locality. — Lower Cambrian : red sandy limestone of 

 Woodlands Quarry, Hartshill. 



Hyolithus (H.) Willsi, sp. nov. (PI. IV, Figs. 17, 18.) 



? Hyolithus cf. princeps, Billings, Lapworth, op. supra cit., pp. 343, 349. 



The type-specimen [27 and 28] exhibits a fragment of the dorsal 

 face together with (on the other side of the piece of limestone) the 

 much-weathered ventral face, from which the outline of the shell 

 may be reconstructed; two oblique sections are also visible. 



Diagnosis. — Shell large, straight, tapering at the rate of 1 in 2'6 

 (equivalent apical angle 22°). Section uncertain (the specimen 



1 U.S. Geol. Surv., Bull. 10, 1884, p. 20, pi. ii, 



2 Holm, op. cit., 1893, p. 78, pi. i, figs. 72-7. 



3 Id., p. 77, pi. v, figs. 23-8. 



l. 5. 



