Notices of Memoirs — E. A. N. Arber — Plants as Zonal Indices. 359 



posterior end vertically subtruncate at its mid-heiglit, rounding 

 abruptly into the cardinal margin above and into the ventral margin 

 below. Ventral margin gently convex, but curving upward more 

 abruptly and rapidly at the posterior than at the anterior end ; 

 superior border almost straight and nearly horizontal ; umbones 

 depressed, anterior, very nearly but not quite terminat; beaks 

 incurved. 



Surface -markings not at all well preserved in either of the 

 specimens collected, but apparently consisting of fine concentric 

 lines of growth. Hinge dentition and muscular impressions un- 

 known. 



Approximate dimensions of the specimen figured : maximum, 

 length, 15 mm.; greatest height, 11 mm.; maximum width, or thick- 

 ness through the closed valves, nearly 9 mm. 



Trenton Limestone, Ottawa, E. Billings : four nearly perfect but 

 badly preserved specimens. 



M. brevis can be distinguished at a glance from M. tener, M. rugosa, 

 and M. recta, by its comparatively short, tumid, and regular convex 

 valves. 



III. — The use of Carboniferous Plants as Zonal Indices. By 

 E. A. Newell Arbkr, M.A., F.L.S., F.G.S.^ 



rSlHE student of Carboniferous plants has long ago realized that 

 L the kind of evidence which is drawn successfully from the 

 distribution of marine invertebrata is inapplicable and inaccurate 

 in the case of fossil plants. For instance, as is well known, 

 the Jurassic rocks are divided into a number of zones on the 

 occurrence of a species of Ammonite, confined or almost wholly 

 confined to a particular zone. Apparently in regard to the Carboni- 

 ferous mollusca, the same principle is being applied. Efforts are 

 being made " to obtain one or two definite but common mollusca 

 which occur in one subdivision of the Coal-measures, but which are 

 absent or almost entirely absent from others, and to use such species 

 as zonal indices. How far this will prove possible in the case of 

 a fauna which has for the most part a wide vertical range, and which 

 is not truly marine but largely littoral, estuarine, or even fresh-water, 

 remains to be seen. The discovery of restricted species of plants 

 is not, however, the primary object of the paleeobotanist. Some 

 geologists, realizing that fossil plants do not commonly afford this 

 type of evidence, have rather hastily concluded that such remains 

 are therefore useless as zonal indices. I hope, however, to show 

 that this is not the case. It is true that in British rocks a number 

 of plants are, so far as our knowledge extends, confined to one of the 

 minor divisions of the Carboniferous, such as the Middle Coal- 

 measures. This is the case with Zeilleria delicatnla (Sternb.) and 



1 Abstracted from a paper read before the North of England Institute of Mining 

 and Mechanical Engineers in June, 1903, and published in the Transactions of the 

 Institute for the current year. 



2 "Life-Zones in the British Carboniferous Eocks : Eeport of the Committee " : 

 Eeport British Association, Bradford, 1900, pp. 340-342. 



