146 SYNOPSIS OF THE CHARACTERS OF THE 



This is tlie only common species of tlie genus Br achy thy r is. It is, perhaps, sufficiently described in the 

 above character : I may add, however, that Sowerby considered a sulcus down the middle of the mesial eleva- 

 tion, and the absence of sulci in the mesial hollow, to be important characters ; the mesial ridge is, however, 

 perhaps oftener found without a mesial sulcus than with, and the mesial hollow frequently exhibits obscure 

 sulci, or traces of ribs. Bronn has confounded the present shell with the Spirifera ostiolata of the Eifel ; it 

 is, however, distinguished at a glance by its short hinge-line and approximate beaks. I have not quoted Pro- 

 fessor Phillips' figure for this species, as that appears to me to be the young of Spirifera rotundata, as is indeed 

 suggested by himself in the text. Many authors have considered this shell to be the young of Spirifera. rotun- 

 data, but it may be distinguished from all ages of that species by its width never exceeding its length, its greater 

 depth, and its consequently obtuse edge. From Brachythyris integricosta it is known by its want of plaits on 

 the mesial fold. Length one inch. 



Bkachythyris planata. Phil. sp. 



Anomites rotundata. Martin, Pet. Derb — Spirifera plauata. Phil. Geol. York. 



Sp. Ch. — Orbicular ; ventral valve plane ; radiations numerous, obtuse. 



The almost perfect flatness of the lesser valve of this species forms one of its most remarkable peculiarities. 

 The longitudinal ribs are flattened ; there are three on the mesial fold, and eleven or twelve on each side. It 

 appears to me that this is the shell intended by Martin as Ms Anomites rotundatus, and not the Spirifer rotun- 

 datus of Sowerby ; the small size, circular outline, flatness of the smaller valve, and want of the mesial elevation, 

 agreeing much better with the present fossil, than with Mr. Sowcrby's species. Professor Phillips' name is, 

 however, so well established for this species, and the shell described in the Mineral Conchology, from being one 

 of the most abundant mountain limestone fossils, is so well known by Mr. Sowerby's name, that to transpose 

 their specific appellations would be productive of more inconvenience than the mere establishing the priority of 

 Martin's name for the present species would perhaps warrant. Length one inch, width the same. 



Brachythyris PLANicosTATA. M'Coy. (PL XXI. fig. 5) 



Sp. Ch. — Shell transversely oval, gibbous, rounded; mesial elevation large, square, defined ; entire surface 

 radiated with numerous, narrow, smooth, flat ribs. 



This species resembles the Spirifer exaratus of Fleming, in the singular character of the two valves being 

 almost invariably found apart, and, I might add, in the great rarity of the smaller valve in most localities. From 

 the dark limestone at Killymeal, Dungannon, I have seen many specimens of the large valve, which is very com- 

 mon there, while I have seen but one or two examples of the small valve ; while, on the other hand, in the 

 limestone of Mullaghfin, Duleek, the two valves are equally common, and almost always found in contact. The 

 specimen figured is from this latter locality. Length two inches, width three inches. 



Athyris. M' Coy. 



Gen. Cli. — Nearly orbicular, small; no cardinal area or hinge-line; spiral appendages very large, filling 

 the greater part of the shell. 



This very interesting group possesses all the external characters of the Terebratulida, united to the inter- 

 nal structure of the Spirifers, to which latter family it truly belongs. Professor Phillips is the only author 

 who has recognized the group ; he forms of it his last division of the genus Spirifera, but gives no characters 

 to distinguish it from Terebratula; the internal structure is, however, a sui'e guide. 



