CARBONIFEROUS FOSSILS OF IRELAND. 149 



Tliis is one of the most elegant shells of its genus, and is rendered particularly striking by its erect laminai 

 of growth; these arc usually about nine in number, perfectly continuous across the shell, and, when well pre- 

 served, stand a line or more in height*, perpendicular to the surface of the valve, they arc about a line and a 

 half distant from each other, the spaces between them being perfectly flat and smooth. The mesial rid"e, 

 though narrow, is rounded and very prominent at the margin, but becomes obscure towards the beak of the ven- 

 tral or smaller valve ; the mesial sulcus on the dorsal or larger valve is, however, sharply defined, and continues 

 distinct up to the beak. Length usually one inch, width about one-fourth greater than the len<'-th, depth one- 

 half the width. 



AtHYRIS (?) TRILOBA. M'Coif. (PI. XX. fig. 21). 

 Sp. Ch. — Longitudinally ovate, subtrigonal; mesial fold very broad, prominent, slightly convex, divided 

 by a faint mesial sidcus ; sides rounded, narrower than the mesial fold ; surface regularly marked by strong, 

 transverse, imbricating laminse of growth. 



This shell is so distinct from the other Brachiopoda of the formation, that it is not likely to be confounded 

 with any other species ; the great width of the mesial fold is very remarkable ; the surface is smooth, with tlie 

 exception of the regular, thick lamina of growth. Length five lines, width six lines. 



AcTiNocoNCHUs. M Coy. 



Gen. Ch. — Shell globose; the margin of both valves greatly extended, forming a flat, circular, stri- 

 ated disc ; spiral appendages as in Athyris. 



I founded this genus several years ago, in a paper read to the Geological Society of Dublin, for the recep- 

 tion of a very singular fossil, occasionally found in the lower members of the carboniferous series. Specimens 

 such as are usually found, present the appearance of some globular, smooth species o? Atrijpa which had by ac- 

 cident fallen on a portion of an Orthis Pecten, or some such fossil; the discrepancy between the smooth, 

 globose shell, and its flat, striated margin, being so great, that few would imagine they were parts of the same 

 species. The discovery of the specimen figured, PL XXI. fig. 6, a, first led me to consider them as parts of the 

 one shell, by exposing the upper valve perfect, the beak of the lower valve, and the flat, striated laminse ap- 

 pearing from between them ; but it was not until I received the specimens, fig. 6, b, that the structure of the shell 

 was at all clear. 



The central globular portion resembles a smooth Terehratula, or Atrypa, with a few concentric lines of 

 growth, but the dorsal valve, as far as I can judge from the specimens I have examined, is quite imperforate; 

 the few concentric lines of growth I imagine to be the remains of disciform margins to the valves of the young 

 shell, such as are now seen in the old ; and as the shell advances in growth, the two flat margins, instead of 

 being in contact, and aflbrding each other mutual support, become erect, insulated, scale-like lamina;, liable to 

 be broken oft' by the slightest accident; so that each line of growth, or former margin of the shell, might have 

 been dilated, as in the example figured, as long as it was the margin ; but when a new edge was formed be- 

 tween the two old discs, they fell off as superfluous, like the tip of Buliinus decuUatus, merely leaving a concen- 

 tric line of growth to mark their places. The same thing can be seen in such shells as the Tridacua squamosa, 

 where those scales near the beak are always effaced, and thus it is that we only find the two last formed discs 

 on the present shell ; occasionally, however, we find portions of the old margins, which, having been supplanted, 

 are in a state of decay ; a specimen of this kind is figured, PI. XXI. fig. 6, a, in which two discs are visible on 

 each valve, the one broken and in a state of decay, the other projecting far beyond it. 



Since writing the above I have seen a specimen from the Cork limestone, in which the laminse nearest tlie edge 

 were prolonged nearly an inch from the surface of the shell, and radiatingly furrowed, as in the Actinoconchtts paradoxus, 

 M'Coy; this latter may therefore be an Athi/ris. 



2P 



