ESTHERIA MIDDENDOREII. Ill 



sculpture sometimes seen in E. ovata (PL II, fig. 33) ; but the bar-ornament of the 

 latter is wanting. 



E. Forbesii, though considerably larger, has much the same outline as some individuals 

 of E. minuta (PL I,- fig. 29) ; it has the same distinctness in its ridge-growth, and the 

 same kind of ornament (the pattern being relatively smaller than that of the typical form, 

 and scarcely larger than that of the Rhaetic variety). Still, its much larger size, and its 

 suborbicular form in the immature state, together with its occurrence in the other hemi- 

 sphere, are sufficient to keep it specifically apart. The fact, too, that the reticulate orna- 

 ment, but slightly modified, occurs on several modern as well as ancient species, must 

 make us careful in applying it as a specific character either of alliance or distinction. 



The geological position of the Estherian shale under notice is doubtful, and the 

 Est/ieria itself affords no certain clue to its determination. The nearest ally (though 

 sufficiently distinct) is E. ovata of the North American continent. Through possibly of 

 Lower Mesozoic age, still the shale may belong to the Upper Mesozoic, or even to the 

 Tertiary, period, so isolated is the place of deposit, and so manifold are the alliances of 

 the animal, as far as the characters of the carapace are concerned. 



14. Estheria Middendorfij, Spec. Nov. PL IV, figs. 12 — 22. 



Height of valve % inch ") „ . 



T 6 , r Proportion 3 to 5, or 1 : 1 | + 



Length £ „ j l «; 



Carapace- valves thin, suboblong, straight on the dorsal margin, nearly the whole of 

 which is occupied by the hinge-line ; umbo forward, not preserved in the many specimens 

 seen; ends well rounded, and nearly equal; ventral margin FlG n 



gently and nearly symmetrically curved. Ridges distinct, about 

 twenty-four, sometimes more numerous, and crowded towards 

 the ventral edge ; interspaces bearing an open, irregular reticu- 

 lation (fig. 1G), often passing into thin, transverse, somewhat EMa md ^ dorJiii (nm 

 irregular riblets (fig. 15); the irregularly hexagonal areas of Siberia. Natural size. 

 the reticulation, when highly magnified, are seen to be delicately punctured (fig. 19). 



This fine Estheria, one of the largest known, occurs fossil in Siberia ; it has been 

 noticed by Dr. A. Th. von Middendorf (see further on). Most of the specimens that I haw 

 seen were brought to England by Mr. C. E. Austin, E.G.S., and are doubtless similar in 

 every respect to those alluded to by Von Middendorf. They consist of a bluish-grey, 

 finely laminated shale, slightly micaceous, indurated, fissile, and easily broken into 

 irregularly shaped pieces. Some of the shale has the surface-planes of all the laminae 



