ESTHERIA MEMBRANACEA. 19 



The Livonian specimens are well preserved in their impalpably fine-grained matrix ; 

 they are much flattened, of a dull honey-colour, and beautifully neat in their graceful 

 outline and their delicate concentric ridges (PI. I, fig. 6). Like the Caithness specimens 

 (PI. I, figs. 1 — 4), they are almost symmetrically semicircular on the front, ventral, and 

 hinder borders, and straight along the whole dorsal line, except where the umbo slightly 

 protrudes somewhat in front of the centre. The outline is somewhat quadrate, with a slight 

 obliquity, due to the eccentric umbo being the starting point of the conforming concentric 

 ridges. The fulness of the curves of the ridges and of the ventral border are somewhat 

 hindwards ; the ridges being closer together on the anterior than on the posterior portion 

 of the valve. 



The well-preserved condition of the carapace-valves enables us to recognise about 

 thirteen concentric ridges ; and in some specimens a few fainter intermediate strise are seen 

 under the microscope. The coarse rounded wrinkles of the specimens in the Caithness 

 sandy flagstones are the rough modifications of this delicate structure ; either the ridges 

 being squeezed up and distorted, or, in some cases, the intermediate hollows having been 

 swollen up by the wrinkling pressure of the sandy matrix (figs. 2, 3, and woodcut, p. 22). 



In the Livonian specimens, the interspaces between the ridges appear to be delicately 

 sculptured with faint transverse wrinkles (fig. 7). This is in strong contrast to the coarsely 

 granular appearance of the Caithness specimens (fig. 5), which have probably been 

 impressed with the sand-grains of the matrix. 



Jsmusia membranacea, Pacht, is mentioned by Dr. Ch. Pander in his memoir 

 ' Ueber die Saurodipterinen, Dendrodonten, Glyptolepiden, und Cheirolepiden des 

 devonischen Systems' (St. Petersburg, 1860, p. iv), as being found in a greyish, 

 laminated, calcareous marl, with intercalated grey and bluish clays, both full of Devonian 

 fish-remains, on the River Torgel, in Livonia ; but I was not aware that it had been figured 

 and described until I received Dr. Ch. Pander's communication in November, 1861. (See 

 p. 21). Of Kokenhusen, the locality above mentioned for A. membranacea, we have the 

 following particulars in the ' Geology of Russia,' &c, by Murchison, De Verneuil, and 

 Von Keyserling (p. 51) : 



" The picturesque rocks in the environs of the Castle of Kokenhusen [in Livonia] 

 particularly deserve notice, not merely on account of the thickness of the vertical section 1 

 (speaking, of course, by comparison), but specially because the beds contain Ichthyolites. 



Palaeontologie Dorpats,' &c, 1835 — 37 ('Verhandl. d. min. Gesellsch. zu St. Petersburg,' 1846, p. 110, 

 pi. 7, fig. 12) ; and is also described in Eicbwald's ' Lethsea Rossica,' livr. 6, p. 921. It occurs in tbe Old Red 

 Sandstone series, near Dorpat, in Livonia, on the borders of the River Oredege, near Gatschina, and in the 

 micaceous argillaceous limestone co-ordinated with the Old Red Sandstone, at Sivoritzy, near Gatschina, in 

 the Government of St. Petersburg. Lingula also accompanies Holoptychius and Pterichthys, in Devonian 

 marls and sandstones, underlying the Coal-measures, in the Gouvernement Nowgorod, on the Prykscha, near 

 Scherechowitschi. (G. von Helmersen, 'Mem. Acad. Imp. Sc. St. Petersb.,' vol. iii, No. 9, p. 22, 1860. 



1 In the Index to the 'Geology of Russia,' &c, the word "vertical" before "Devonian beds con- 

 taining Ichthyolites" should be erased, as the beds are almost horizontal. 



