ATHYRIS. 15 



Terebrattjla hispida, Sow. Trans. Geo]. Soc, 2nd series, vol. v, pi. liv, fig. 4. 



— concentrica, De Verneuil. Bull. Soc. Geol. de France, vol. xi, p. 251, 



pi. ii, fig. 1, 1840. 

 Spirigera concentrica, D'Orb. Prod., vol. i, p. 98, 1849. 

 Athyris concentrica, M'Coy. British Palaeozoic Fossils, p. 378, 1852. 

 Spirigera — Bav. General Introduction, vol. i, pi. vi, figs. 65 — 67, 1853. 



Terebrattjla — Schnur. Beschreibung Eifel. Bracliiopoden, iu Palaeoutographica, 



vol. iii, p. 3, 1853. 

 Athyris — Morris. A Catalogue of British Fossils, p. 130, 1854. 



Spirigera — Sandberger. Die Bracliiopoden Rheinischen Schichten-Systems in 



Nassau, p. 31, pi. xxxii, fig. 11, 1835. 



Spec. Char. Shell usually wider than long, suborbicular, transversely oval, circular, 

 or slightly subpentagonal, with rounded contour ; valves almost equally convex, increasing 

 in gibbosity with age. When young, the front is evenly rounded, but with age a more or 

 less developed broad lobe or mesial fold commences at a little further down than the 

 middle of the valve, and extends to the front, near to which it attains its greatest elevation. 

 In the ventral valve a sharply defined sinus commences towards the middle of the valve 

 and extends to the front. Beak tumid, moderately produced, incurved and truncated by 

 a small circular aperture lying close to the umbone of the opposite valve ; beak-ridges 

 undefined, surface more or less deeply marked by numerous close, concentric, regular, 

 imbricating laminae of growth. In the interior of the smaller or dorsal valve the hinge- 

 plate presents four depressions or pits, and close to the extremity of the umbo a small 

 circular aperture appears to commuuicate Avith a circular tube, which, after originating 

 under the platform, extends longitudinally and freely with a slight upward curve to about 



is not unfrequently shown in the cleavage of the beak of that valve, in solid specimens, where the interior 

 is inaccessible. 



"The forms which I have regarded as Merista are similar to those above ; but instead of this septum, 

 or shoelifter process, they have a deeply marked, triangular, muscular area, just below the rostral cavity of 

 the ventral valve, which is bordered on the anterior side by a callosity of the shell, and on the two other 

 sides by the strong dental lamellae. This feature is not conspicuous in Athyris ; the dental lamellae in that 

 genus are shorter and less strong, and the form of the muscular impression is different. The dorsal valve 

 of those shells now under consideration has a longitudinal median septum, a feature which is obsolete, or 

 partially obsolete, in the species of Athyris. In the Camarium or Merista proper the exterior of the ventral 

 valve sometimes shows what appear to be two diverging septa, somewhat similar to those in the dorsal 

 valve of Pentamerus, which are the margins of the shoelifter process 



" Restricting, therefore, the signification of the genus Merista to such forms as were originally 

 included by Professor Suess under that name, it becomes necessary to designate those species of similar form, 

 but without the peculiar appendage of the ventral valve, by another generic term, and I would therefore 

 suggest the name Meristella, proposed by me last year." These three genera or sub-genera would therefore 

 be typified as follows : 



1. Athyris, or Spirigera. Ex. A. concentrica, A. phalama, &c. 



2. Merista. Ex. M. Herculea, M. plebeia = M. scalprvm, &c. 



3. Meristelea. Ex. M. tumida, M. Icevis, M. bella, &c. 



