150 E. S. Cobbold — Cambrian Hyolithidce from Hartshill. 



and, where they have the same orientation in cross section and are 

 closely approximated, they give the appearance (noted by Billings 1 

 for H. communis) of the individual shell being "thickened by 

 concentric laminae, and thus approaches the structure of Salterelia". 

 A few detached opercula occur, usually very indifferently pre- 

 served, and it is impossible to say, without reserve, to which species 

 they belong. 



Descriptions of Species. 



HYOLITHUS, Eichwald. 



Sub-genus Outhotheca, Novak. 



Ryolithus ( Orthotheca) de Geeri, Holm. (PL IV, Figs. 1-6 and (?) 7-9.) 



Hyolithus (O.) de Geeri, Holm, Sver. Geolog. TJndersokning, ser. C, No. 113, 



p. 54, pi. i, figs. 25-7, 1893. 

 Orthotheca de Geeri, Holm, Lapworth, Proc. Geol. Assoc, vol. xv, p. 345, 1898. 



Shells referred to this species are very plentiful in the collection. 

 They agree closely with Holm's figures and description, the only 

 difference observed being that the apical angle varies from 12° to 8° 

 (taper 1 in 5 to 1 in 7) instead of #rom " 9° to 8° ". 



The ratio of the sectional diameters also varies somewhat, but is 

 always near to 3 to 2. 2 The surface has a texture as of ground glass, 

 with very faint striae of growth, which are transverse on both dorsal 

 and ventral sides. The aperture is frequently seen, but in no instance 

 has the apex been preserved. In two cases the internal cast has 

 a smooth convex end representing a septal division below which the 

 shell has been apparently decollated. 



Operculum. — A. few examples of opercula occur that are referred 

 with reserve to this species. In outline they agree with the 

 transverse section of the principal shell ; the margin is practically in 

 one plane ; the nucleus is situated at about two-thirds of the diameter 

 from the dorsal edge ; the semi-conical portion is clearly marked off 

 by two radiating lines, and in addition to these, on the exterior 

 surface, two other radiating lines are seen, marking off triangular 

 portions on each side similar to those of the opercula of Hyolithus, 

 sens, strict., at the places where the curvature changes from the 

 conical (dorsal) portion to the upturned (ventral) part; at the bases 

 of these triangles the margin is seen in a side view to be a little 

 depressed from the general plane ; the ventral margin is marked by 

 a slightly raised, rounded fillet, between which and the nucleus there 

 is a pit-like hollow. 



JET. (0.) Johnstrupi, Holm, 3 has a cross section that is somewhat 

 similar, but the dorso-ventral diameter is proportionately greater and 

 the surface marks are described as being rather strong. 



M. primcevus, Groom, 4 from Malvern, has a very similar cross 

 section but a rather greater apical angle (10° to 11°) and indications 



1 Billings, Canada Naturalist and Geologist, ser. II, vol. vi, p. 214. 



2 Where the ratios of the diameters are mentioned in this paper it is intended 

 to indicate those of the width to the dorso-ventral diameter. 



3 Op. cit., p. 56, pi. i, figs. 28-33. 



4 Q.J.G.S., vol. lviii, p. 116, 1902. 



