110 FOSSIL ESTHERLE. 



(fio-s. 9 and 10). Valve well rounded in front and behind, but somewhat obliquely ; the 

 antcro-vcntral and postero-dorsal margins sloping parallel to each other, and giving a some- 

 what rhoniboidal outline to the full-grown shell. The ventral margin gently rounded in 

 the adult, fully rounded and almost semicircular in the young state. The dorsal margin 

 straight along the hinge-line, which occupies the middle of the border for a distance 

 equal to more than half the length of the valve, and sloping off rapidly before and behind. 

 Umbo distinct, terminating the hinge-line in front, and situated one fifth of the length of 

 the valve from the anterior extremity. Ridges distinct, wide apart, about twenty-two in 

 the adult ; the interspaces ornamented with a delicate, irregularly hexagonal reticulation 

 (fig. 11), with about seventeen meshes from ridge to ridge, and very similar to the orna- 

 mentation of E. minuta, var. Brodieana (PI. II, fig. 15). 



The species under notice is new ; the specimens were collected in large quantities by 

 Mr. David Forbes, F.R.S., F.G.S., three or four years since, at a place called Cacheuta, 

 about 3500 or 4000 feet above the sea, on the eastern slope of the Andes, south of Men- 

 doza ; and I dedicate the species to this adventurous geological explorer of Chili, Bolivia, 

 and Peru, some of the results of whose researches in these regions are published in the 

 ' Quart. Journ. Geol. Soc./ vol. xvii. 



Mr. D. Forbes informed me that he discovered these fossils " in soft beds, together 

 with abundant impressions of ferns, rushes, and reeds." He adds : . " I found no other 

 fossils. The beds are tilted, and in some places much altered by volcanic rocks, and 

 appear to correspond with the beds in Darwin's section of Uspalata Pass, 1 in which 

 he found a fossil forest." 



E. Forbesii occurs in a pinkish-grey, finely laminated shale, indurated, and not breaking 

 evenly along the lines of bedding. The carapace-valves are abundantly strewed throughout 

 the shale, but do not lie very closely together; they are fawn-coloured, sometimes closed 

 and filled with the matrix ; generally separate, but not unfrequently in pairs, with the 

 dorsal edges approximated, and sometimes retaining a considerable amount of convexity. 

 Valves of young individuals are not uncommon among the others. Fragments of Ferns 

 or Cycads and other obscure Plant-remains are scattered here and there. 



The preponderance of the immature and somewhat suborbicular valves of this wide- 

 ridged species on some specimens of the shale reminds one of the Pennsylvanian Estheria, 

 when these have not their ridges crowded up into striae. Moreover, the immature form 

 of the South American species (PI. IV, fig. 9) is much like a youngish individual of the 

 North American E. ova/a (PI. II, fig. 20). They differ, however, in the dorsal angles ; 

 and the adults differ still more in outline, the narrower and exceptional form of E. 

 ovata being scarcely worthy of being taken into account, as its shape may be due to 

 oblique pressure or imperfect exposure of the margins of the valve. The pattern of 

 ornament of & Forbesii (PI. IV, fig. 11) is not widely dissimilar from the reticulate 



1 'Geology of Sowtli America,' p. 202. 



