igi6 No. lo. 



THE LOWER CAMBRIAN HOLMIA FAUNA. 



13 



/ 



A careful study of the extensive material that we have from Tomten 

 has shown that the Norwegian form in certain respects differs from the 

 Swedish, and in all probability must be regarded as a separate species. 



Description: The cpntours of the shell as a rule are almost circular; 

 this is especially the case in the ventral valve, whilst the dorsal valve often 

 has a more oval appearance. However, the breadth is almost always some- 

 -what greater than the length. The shell is slightly pointed posteriorly. 



The beak of the ventral valve is 

 raised above the plane of the shell at the 

 receding and somewhat strongly developed 

 .area, whilst the beak of the dorsal valve 

 is bent down to the plane of the margin. 



The surface of the shell exhibits 

 -certain sharply marked lines of growth 

 <<pl. I, fig. I — 21; between the latter we 

 notice fine and less distinct lines of the 

 same kind. Moreover there are always 

 radiating lines, although as a rule slightly 

 developed. 



The area of the ventral valve 

 is comparatively narrow (in the longitudinal 

 axis of the shell i; this feature, however, 

 appears to vary considerably. The area 

 bends back at an angle of about 30'^ from 

 the plane of the shell. The breadth is con- 

 siderable, and the surface exhibits marked 

 transverse furrows, that at the extremity 

 bend forwards towards the lateral edges 

 of the shell (text-fig. 3). As is always the 

 case, the pedicel tube is plainly visible in 



the cast of the shell, and is narrow, markedly conical, and provided with strong 

 transverse furrows. In one specimen the breadth at the base was ^ u — ^'jo of 

 the breadth of the shell, and the length about \'s of the length of the shell. 



The area of the dorsal valve is more narrow than that of the 

 •ventral valve and is not bent backwards, the beak of the valve at that 

 place, as mentioned above, being bent down the margin. In well preserved 

 specimens this area also exhibits transverse furrows. In the centre we see 

 two furrows, first broad then narrow, converging towards the posterior 

 region (raised in the casts), which form a triangularly bordered region 

 ( fig. 8 and text-fig. 3). 



Fig. 3. OboUlla rotundata nov. sp. 



Reconstruction of interior of shells, X7. 



a Dorsal valve, b Ventral valve. 



