part 4] CAMBRIAN HORIZONS OF COMLET. 331 



Diagnosis. — Ventral valve: outline posteriorly somewhat 

 hexagonal, but anteriorly semicircular ; length and width equal ; 

 widest at about the half-length, posterior border about two-thirds 

 the greatest width, lateral angles well-marked but not sharp ; 

 rather strongly convex ; false area slightly inclined forwards, 

 pseudodeltidium triangular, convex, of moderate width, angle 

 made by its bounding lines about 90° ; shell-substance corneous, 

 not very thick ; exterior marked with concentric ridges, which are 

 irregular and in places present a beaded appearance. 



Internally, or on an interior layer, there are numerous radiating 

 stria?. 



Remarks. — The concentric ridges appear to be rather coarser in 

 this species than in the two previously noticed, but are only known 

 from the marginal portions of the shell. 



Dimensions in millimetres: — 



[King 2 fc] [King 2 g\ 



Length 5*75 ? 



Width 6-75 7-5 



Width at posterior border 5"0 5 - 



Height 2-0 ? 



The species is distinguished by its short posterior border, by its 

 pseudodeltidium being nearly a right-angled triangle, and by the 

 forward inclination of the false area. 



Locality and horizon. — The Cwms sub-area of Comley, Lower 

 Cambrian ; from the Obolella-groomi Beds, horizon Ab r 



These three species of Micromitra appear to be closely related. 

 They occur together in the same beds, but in different numerical 

 proportions. M. ldngi has been found only in the mottled 

 beds, M. phillipsi in the same beds and sparingly in the greenish- 

 grey sandstones, M. rliodesi sparingly in the mottled beds and fairly 

 abundantly in the greenish-grey sandstones. 



They are clearly distinguished one from the other by then- 

 outlines and false areas. The surface-characters seem to differ 

 a little in detail. 



The divergences may be tabulated as follows : — 



M. ~kingi. 



M. phillipsi. 



M. rliodesi. 



Apex (Not known.) Projecting backwards. Approximately vertical.. 



Anterior slope. Decidedly Slightly convex. Straight. 



convex. 

 False area . . . Inclined for- Vertical. 



wards. 

 Angle at apex 1 



of pseudo- I about 90°. about 120°. 



deltidium J 

 Greatest width. At half-length. At quarter- length. 

 Postero-lateral Blunt. Rounded. 



angles. 

 Shell- substance. Moderately Rather thick, 

 thick. 



Inclined forwards. 



about 120°. 



At posterior margin. 

 Sharp. 



Very thick. 



