258 Dr. J. Allan Thomson — The genus Bouchardia, 



a few feet of Carboniferous Limestone at places south of Osgathorpe 

 as shown by a boring at Desford where the Carboniferous Limestone 

 was found to rest on Pre-Cambrian, supply evidence that shallow- 

 water conditions existed in the area during Carboniferous times. 



EXPLANATION OP PLATE XI. 

 MlCROPHOTOGRAPHS OF THE LEICESTERSHIRE DOLOMITES. 

 Fig. 1. — Eed dolomite, Breedon, Leicestershire. Idiomorphic rhombohedra, 

 having central zonal inclusions of haematite and fairly clear outer 

 zones. X 25. 



,, 2. — Red dolomite, Breedon. Calcite of crinoid ossicle invaded by fine- 

 grained haematite-bearing dolomite. Matrix completely'dolomitized. 

 X 25. 



,, 3. — Grey dolomite, Ticknall, south - eastern border of Derbyshire, 

 A mosaic of rather allotriomorphic grains devoid of zonal haematite 

 but having many dusky inclusions. A few rhombohedral outlines 

 are shown. A little limonite is interstitial, x 25. 



,, 4. — Fossiliferous dolomitic limestone, Calke Park, Derbyshire. Typical 

 section showing preference of dolomite for organic structures. The 

 matrix is partly composed of recrystallized calcite. x 25. 



III. — The genus Bovcbardia (Brachiopoda.) and the Age of the 

 Younger Beds of SErMOUR Island, West Antarctic. 



By J. Allan Thomson, M.A., D.Sc, F.G.S., Director of the Dominion 

 Museum, Wellington, N.Z. 



The Age of the Younger Beds of Seymour Island. 



SHELLS with the external aspect of Bouchardia have been known 

 for some time from the New Zealand Tertiary (Oaraaruian), 

 and were first described by Hutton in 1905^ under the names 

 of Bouchardia rhizoida and B. tapirina? The correctness of this 

 generic ascription was doubted by von Ihering, who stated that 

 the shells lacked the characteristic external form of Bouchardia.^ 

 In this, however, von Ihering was mistaken, probably owing to the 

 unsatisfactory nature of Hutton' s figures, for these species agree 

 externally with Bouchardia in the very characters which he supposes 

 they lack, viz., the very sharp beak ridges, the more or less straight 

 sides, and the presence of a longitudinal cord over the suture of the 

 deltidial plates. The most characteristic external feature of the, 

 shell of Bouchardia is that the sharp beak ridges unite in an apex 

 dorsally of the foramen, i.e. the foramen is epithyrid. In Hutton's 

 supposed Bouehardics the foramen is permesothyrid, but almost 

 e])ithyrid. 



Buckman^has described a number of species of Bouchardia from 

 the younger beds of Seymour Island, West Antarctic, and having to 

 rely practically on these alone for a determination of the age of the 

 beds, and finding no help in the way of direct zoological comparison, 

 he has been forced to fall back on a biological argument. 



' Trans. N.Z. Inst., vol. xxxvii, p. 480. 

 " Not Waldheimia tapirina, Hutton, 1873. 

 ^ Ann. Mus. Nac. Buenos Aires, t. xiv, p. 473, 1907. 



* Wissensch. Ergebn. Schwed. Siidpolar-Exped., Bd. hi, Lief, vii, pp. 14-17, 

 32, 1910. 



