33 FOSSIL ESTHERLE. 



scope, one has been carefully figured in PI. I, figs. 22 — 24. It is very small, only g ', inch 

 in length ; but, with great care, it can be made to show the ornament that is highly magnified 

 in fig. 24. I believe that these minute Estheria are of the same species as the larger one 

 on the same piece of mar] ; and that, therefore, two names are not required for them. 

 The larger form is also very delicate, but it is thicker, and of a darker colour than the 

 little specimens marked "Posidonomga Eos '' by M. D'Eichwald ; and the large specimen 

 does not yield good microscopical evidence of ornament. Judging, however, by the 

 probable collocation of large and small specimens of the same species, and by the absence 

 of any important distinction (the relatively great size of the wrinkles being due to modifi- 

 cation of a large specimen by pressure), I must regard both P. exigua and P. Eos as 

 belonging to one and the same species of Estheria. 



In communicating to me the above-mentioned specimens, M. D'Eichwald pointed out 

 that, in his opinion, Estheria Eos has very fine and very numerous concentric ridges, and 

 that " Posidonomga" exigua has strong ribs and broad furrows between them; and, 

 though the former may probably be Crustacean, yet the latter at least, he still thinks, is 

 really Molluscan. M. D'Eichwald also remarked, in his letter of January 19th, 1861, that 

 the fig. 4, pi. 40, of his ' Lethsea Rossica/ represents the ridges as too small; they ought 

 to be stronger and wider apart than in the figure there given. 



The specimens sent by M. D'Eichwald are small pieces of — 1. Light-grey soft marl, with 

 " Bairdia} Pgrrha " and filmy valves of E. Eos, iridescent under the microscope. 2 and 

 3. Dark-grey hardish marl, having the surface-plane covered with delicate crumpled 

 Estherian valves, minute and iridescent, chiefly E. Eos ; a fragment of E. exigua occurs, 

 lying with the others, on No. 2. 



The little Estheria (figs. 22 — 24) from the Permian marls of Russia, has oblong valves, 

 with the ends rounded (fig. 23, magn. 20 diam.) ; the posterior extremity rather more 

 convex than the anterior; the ventral margin slightly curved; the dorsal line straight and 

 long (equal to rather more than half the length of the valve), terminating anteriorly a little 

 in advance of the umbo, which is forward and does not affect the outline of the valve. 

 Concentric ridges about 15, delicate and distinct; the interspaces are neatly punctate 

 with a minute reticular pattern (fig. 24, magn. 100 diam.). 



It is difficult at first sight to find any exact correspondence between the above de- 

 scription of our little valves, and the description given in the ' Bullet. Moscou ' and ' Lethaea 

 Rossica,' of Posidonomga Eos and P. exigua ; nor do our figures agree any better with the 

 figures given by Kutorga and D'Eichwald. Taking in consideration, however, the differ- 

 ence of size of the individuals treated of here and in the works referred to, and the facts, 

 that the coarse ridges and furrows of wrinkled individuals have been described as if they 

 were of the same value as the delicate ridges and their neat interspaces, — that unless 

 drawn with the camera lucida and a good microscope, the niceties of outline and ornament 



1 This is termed "Bairdia" by M. EicWald ; but I see no reason for referring it to that sub-genus. 

 More probably, it is a Ci/theropsis. (See further on, Appendix.) 



