APPENDIX. 343 



PhiUipsii must consequently be regarded as a synonym. Billings' name claiming priority 

 of publication. Dr. HoU informs me that the Potsdam Sandstone and Shale, in which 

 K. cinx/ulata occurs, occupy, he thinks, as nearly as possible the position of our " Lingula 

 beds," and all along the Apalachian chain, rest, like our Holybush Sandstone, unconform- 

 ably on old metamorphic rocks, resembling precisely, and occupying the same position as, 

 those of the Malverns. Dr. Holi suggests the upper portion of the Middle Lingula-flags 

 as the stage at which Kutorgina cincjulata occurs. 



Genus — Acrotreta, Kutorga^ 1848. 

 (See 'General Introduction,' p, 133.) 



Acrotreta (?) Nicholsoni, Dav. PI. XLTX, figs. 36 — 40. 



Acrotreta (?) Nicholsoni, Bav. Geol.Mag.,vol. v,p. 313,pl. xvi, figs. 14 — 16, 1868. 



Sjpec. Char. — Shell small, about two lines in length, by about the same in breadth . 

 almost circular ; rather wider and broadly rounded anteriorly, nearly straight posteriorly ; 

 dorsal valve very slightly convex ; ventral valve conical; apex subcentral and truncated 

 by a minute circular foramen, situated at a little more than one third of the length of the 

 valve. Prom the centre of the posterior margin a narrow groove or channel extends to 

 the base of the foramen, while on either side a small, flattened, triangular space or false 

 area (?) is limited by an indented line. Surface of both valves marked with numerous 

 concentric lines of growth. 



Ohs. This is another of those minute and curious forms that have been discovered 

 subsequent to the publication of the first portion of this ' Silurian Monograph.' Several 

 examples of this interesting little species were sent to me in February, 1867, by Dr. H. A. 

 Nicholson, their discoverer, with the designation of " Siphonotreta micula," but I soon 

 perceived that they could not be identified with that genus or species ; and, although I 

 am by no means confident as to the propriety of placing this form in Acrotreta, this 

 latter is the genus to Avhich our Scottish shell under notice seems to bear the closest 

 resemblance. One valve is nearly flat; the other conical, with a minute perforation at its 

 apex, a longitudinal groove, and a " false area" — all being features (external) peculiar to 

 the Russian genus. However, as we know nothing of the interior of Acrotreta, and very 

 little of the Scottish shell, the true generic position of this species must be viewed as 

 provisional. 



Position and Locality. Acrotreta Nicholsoni occurs in black shales of the Upper 

 Llandeilo (?) beds in Dobb's Linn, near Moffat, in Dumfriesshire ; whilst Siphonotreta 

 micula is found at Hart Pell, or Glenkiln Burn, near Mofi'at, in distinctly different rocks. 

 Sijjhonotreta micula is, strictly speaking, a " Llandeilo flag" species ; but Mr. Hicks 



