WHITEAVES] DEVONIAN FOSSILS, MACKENZIE RIVER BASIN. 233 



EaTONIA VARIABILIS. (N. Sp.) 



Plate 29, figs. 6, 6a, 7, s, 8a and 9. 



Shell compressed biconvex, the thickness through the closed valves 

 varying in different specimens from a little more to a little less than 

 one half of their greatest breadth, transversely subelliptical, a little 

 broader than long : front margin truncated in the centre in some speci- 

 mens (as in tig. 6), regularly rounded in others (as in tig. 7), and in 

 some (as in fig. 8), produced into a short rounded lobe. Ventral valve 

 modei-ately convex on the umbo, immediately' belovr or in fvimt of 

 which there is a more or less well defined mesial sinus, which widens 

 rapidly forward and outward. Beak of the ventral valve small, pro- 

 jecting veiy little beyond that of the dorsal, slightly incurved, 

 truncated and perforated by a minute subcircular foramen, underneath 

 which there is a very small deltidium. Dorsal valve differing little 

 from the ventral in contour, except that it has a mesial fold instead of 

 sinus, and a smaller and entire beak. 



The finer surface markings are very imperfectly pi-esei'ved, nearly 

 the whole of the outer layer of the test of the specimens collected being- 

 exfoliated. Both of the valves, however, are marked with radiating 

 plications, which vary in shape, number and disposition in different 

 individuals. Thus, in the specimen represented by fig. 6 there are 

 three strongly developed subangular plications in the ventral valve, one 

 in the centre of the sinus and one forming each of its outer limits, and 

 four on the dorsal, i.e., two on the mesial fold and one on each side of 

 it. All of these plications, in this as in other specimens, are obsolete 

 on the umbo of each valve, and mo.st prominent around the front mar- 

 gin. In the original of fig. 7 the two plications on the mesial fold of 

 the dorsal valve trifurcate before reaching the front margin, and there 

 is a very short marginal fold intercalated between them. There are 

 three faint but continuous plications on each side of the fold and sinus, 

 80 that round the front margin of this specimen as many as thirteen 

 plications can be counted on the dorsal valve, and there are eleven on 

 the ventral. Lastly, in yet another specimen (fig. 8) there are two 

 rather broad, rounded and continuous plications on the mesial fold of 

 the dorsal, and three in the sinus of the ventral (one in its centre and 

 one forming each of its outer boundaries), but there are no lateral pli- 

 cations. Upon the only specimen in which any portion of the outer 

 layer of the test is preserved, the surface of the latter is seen to be 

 marked by narrow radiating raised lines of very small but unequal 

 size, which are crossed by narrower concentric raised lines, the latter 



