ESTHERIA MEMBRANACEA. 21 



" In the marls near Kokenhusen on the Pehrse, which lie on the lowest Devonian 

 sandstone, form the lowest member of the Devonian limestone, and are particularly rich in 

 the remains of Osfcolepis, Dijjlexus, Asterolepis, &c, occur little bivalves, often in immense 

 quantities, which Eichwald, in his ' Geognosie von Russlancl,' published (in the Russian 

 language) in 1846, p. 339, referred to Posidonomya. 



"Pacht, unaware of this determination, formed of these bivalves a new genus, which he 

 called Asmusia ; and the species, on account of the thinness of the shell, he called membra- 

 7iacea ; 'Der devonische Kalk in Livland,' 1849, p. 44. 



"Afterwards, finding that this animal had been already referred to Posidonomya, he 

 gave up his new genus, in his memoir ' Ueber Pimerocrinites oliyopti/us,' 1852, p. 26, and 

 kept the name Posidonomya. In the same year this species appeared as Posidonomya rugosa 

 in Kutorga's ' Geognostische Karte des Gouvernements von Petersburg,' and as Posido?w- 

 mya membranacea in Pacht's new edition of his ' Der devonische Kalk in Livland,' 1859, 

 p. 44, where it is accurately described, and illustrated by fig. 7 on the plate accompanying 

 his memoir, but not with exactness. 



" On account of Rupert Jones's researches on Estheria minuta in 1855, by which I 

 was made aware that Posidonomya minuta of the Trias is a Crustacean, I examined 

 carefully under the microscope this so-called Posidonomya, and found clearly that its tissue 

 corresponded, not to that of the Acephala, but to that of the Crustacea. I considered, 

 however, judging from its outer form, that this creature could not be allied to Estheria, 

 from which it differs by its straight hinge-border ; and I was therefore uneasy in retaining 

 for it the name given by Pacht. In this mind I have treated of Asmusia membranacea 

 as a Crustacean, and not as an Acephalous Mollusc. 



" This animal is clearly identical with that figured by Hugh Miller in his ' Old Red 

 Sandstone,' pi. v, fig. 7. Should this be the case, it would, as 1 have formerly indicated, 

 contribute much to the correlation of the Caithness shales, so rich in fish-remains, with 

 the marls of Kokenhusen, which have an equally rich ichthyological fauna." 1 [Christian 

 Pander, fjg, 1861.] 



Asmusia membranacea from the sandy clay of Kokenhusen, is the same as the 

 Estheria from the harder and more coarsely grained flagstones of Caithness. In the fine- 

 grained deposit of Kokenhusen, the shell is very thin and of a light amber-colour, and 

 being much less affected by the pressure of sand-grains, though often much crumpled, 

 exhibits here and there faint traces of a regular ornament of a vertical wrinkly reticulation 

 between the ridges (fig. 7). Owing to the difference in the matrix and mode of preserva- 

 tion, in the Russian specimens, the thin upstanding concentric riblets, are better preserved 

 than in the flagstones of Caithness, and therefore form a prominent feature, which in the 



1 A neatly executed sketch, in water-colours, of the Kokenhusen fossil has been kindly communicated 

 to me by Dr. Pander (through Col. von Helmersen), agreeing very closely with our fig. 6 (drawn under the 

 earner a-lucida), but showing eighteen, instead of about fourteen ridges. The sketch also indicates the 

 shorter and the longer varieties of form, such as we have observed among the specimens from Caithness 

 (compare figs. 2 and 3). 



