part 4] 



CAMBRIAN HORIZONS OF COMLET. 



343 



visible, and points upwards and backwards at an angle with the 

 plane of the valve-margin ; on each side of it obtuse prominences 

 project horizontally backwards from the convex body of the cast ; 

 at about a third of the length of the valve from the apex two 

 comma-shaped prominences are situated, having their points 

 directed forwards and nearly meeting at the centre-line ; beyond 

 the swollen ends of the commas, ridges extend forwards and out- 

 wards, and between them and the centre-line, ojfcher ridges, one on 

 each side, may sometimes be detected running in parallel directions ; 

 traces of a backward prolongation of the outermost pair of ridges 

 are occasionally observed (see PI. XXII, fig. 9). 



In the cast of the dorsal valve (PI. XXII, fig. 7 & text-fig. 1) a 

 central depression usually extends from the posterior border to about 



Fig. 1. — Obolella atlantica, var. comleyensis nov. x 10. Dia- 

 grams of infernal casts of the ventral and dorsal valves, 

 showing all marks observed in the several specimens. 



Ventral 



Dorsal 



half the length of the valve ; on each side raised lines diverge 

 forwards, and are traceable for about the same distance ; from the 

 back of the convex body of the cast obtuse prominences project hori- 

 zontally backwards, similar to those of the ventral valve but nearer 

 to the centre-line ; beyond these the body of the c,ast is hollowed 

 out, the hollows being terminated laterally by projecting elongated 

 bosses, more or less parallel to the margin of the valve. 



In both valves a wide flattened or even recurved space extends 

 all round the convex body of the cast. 



All the internal marks that have been observed in various speci- 

 mens have been combined in the outline restoration (fig. 1, above). 



The shell-substance is calcareous. 



Dr. -J. Kiser, in his work on the Holmia fauna of Tomten, 

 describes and figures a species, O. rotundata. that is, at first sight, 

 very like the variety comleyensis. The general form is almost 

 exactly the same, and the resemblance between the dorsal valves is 

 particularly striking. 



The Tomten shell is rather more pointed posteriorly than the 

 Comley form ; it is about 2o per cent, larger in linear dimensions ; 



2b2 



