108 



BRITISH SILURIAN BRACHIOPODA. 



by the umbo of the dorsal valve, or by a deltidium ; area none ; valves articulated by 

 teeth and sockets. Surface smooth, or marked by fine concentric lines of growth (not 

 laraellose), and indistinct or obsolescent radiating striae, which are usually more con- 

 spicuous in the cast or exfoliated surfaces than on the exterior. Shell fibrous. The 

 ventral valve is much thickened on each side towards the beak, and the rostral cavity 

 margined by flattened dental lamellae, which extend downwards to the commencement of 

 the muscular impressions, and terminate at the edge of the shell in blunt, tooth-like pro- 

 cesses. The muscular impression forms a somewhat broadly triangular depression in the 

 valve, just below the rostral cavity. In the cast of this valve we have the reverse of these 

 features. In the dorsal valve there is a strong hinge-plate or process, the prominent part 

 of which is broadly triangular, somewhat depressed or spoon-shaped in the centre, and 

 supported below by a median septum, which reaches from one third to one half the length 

 of the valve, and on each side marked by deep dental fossets, while the anterior angles 

 are produced into the crura which support the internal spires. Spires arranged as in 

 Athyris and Merista, being a double cone with the apices directed outwards. From the 

 lower lateral margins of the cardinal process or hinge-plate there is a callosity extending 

 beneath the anterior to the dental fossets, and joining with the thickened margin of the 

 valve, as in the other allied genera. 



" In the cast of the dorsal valve we have the mark of the median septum, with an 

 elongated lanceolate muscular impression, reaching nearly to the middle of the valve. 

 The imprint of the triangular process, and the cavities made by the crura, are often pre- 

 served The difference between Athyris { = Spiriyera) and Meristella are everywhere 



clear and unmistakable, in the external lamellose siu'face of the one and the almost smooth 

 character of the other. The muscular impressions of the ventral valve of Athyris are 

 at once distinguishable from those of Meristella. In the dorsal valve, the muscular 

 impressions differ from Meristella, the hinge-plate is of somewhat different character, 

 and the median septum is scarcely developed."^ 



I must, however, hasten to observe that, from want of sufficient material, it has not 

 been possible for me, in some cases, to determine with absolute certainty whether some 

 of the species should be placed in the genus Atrypa or Meristella. 



" At pages 282—303 of the fourth vohime of his « Paleeontology of New York' (1857), Prof. Hall 

 describes, at great length, the characters which distinguish Meristella from Athyris ; and he illustrates a 

 series of very remarkable preparations made by Mr. R. P. Whitfield, showing the very complicated manner 

 in which the two spiral coils are attached to each other iu the middle, as well as to the hinge-plate, in 

 various species of the above-named genera. This, though different in detail, bears a certain resemblance to 

 what I described and figured in Athyris pectinifera, at p. 21 of my 'Monograph of British Permian 

 Brachiopoda.* 



