104 SYNOPSIS OF THE CHARACTERS OF THE 



Brachiopoda, to which, in my opinion, it has little immediate aiSnity, both the animal and the shell dliiering 

 in the most important characters ; thus the Bracliiopoda have two hearts, Lingula, like the ordinary bivalves, 

 has but one; all the Brachiopoda are inequivalve, Lhujida equivalve: the Brachiopoda (when attached) have 

 one valve perforated for the passage of the muscle of attachment, in Lingida the muscle of attachment passes 

 between the valves, neither of them being perforated; in Lingula, the gills are attached to the mantle, nearly as 

 in ordinary bivalves, while the true Brachiopoda have no gills at all, the surface of the mantle being covered 

 with vibratile cilice which serve the same purpose ; and many other points of structure, not only in the shell but 

 in the animal, induced me to consider the relation of this genus to the Brachiopoda as less strong than it has 

 been usually considered, and to view it rather as a connecting link between the Brachiopoda and the Dithjra. 



Oebicula. Lam. 

 Gen. Ch. — Depresso-conical, suborbicular ; ventral valve flat, perforated by an oblong fissure for the byssus 

 of attachment ; four muscular impressions in each valve. 



Orbicula nitida. Phil. 



Orbicula uitida. Phil. Geol. York Orbicula nitida. Portk. Geol. Rep — Orbicula cincta. Portk. Geol. Rep. 



Sp. Ch. — Longitudinally oval, slightly narrowed in front, apex about one-fifth tlie length from the ante- 

 rior end ; upper valve convex, lower valve flat, perforated ; surface with concentric, imbricating striae, and very 

 fine, radiating lines. 



1 believe the O. cincta, Portk., may be considered a variety of this species; the radiating striae are very 

 minute. Length seven lines, width six Unes. 



Orbicula quadeata. M'Coy. (PI. XX. fig. 1). 



Sp. Ch. — Subquadrate, very much depressed, length and breadth equal ; beak small, iiiflexed, one-third 

 the length of the shell from the margin; siu'face rough, with concentric, scale-like Unes of growth. 



This species is at once distinguished from the O. nitida, Phil., by its greatly depressed, quadrate form, and 

 its strongly imbricated surface ; the sjDecimen figured was found on a Turbinolia. Length five lines, width the 

 same, height half a Hue. 



Orbicula teigonalis. M'Coy. (PI. XX. fig. 2). 



Sp. Ch. — Conical, obovate, trigonal; anterior end narrow, rounded, posterior subtinmcate ; surface irregu- 

 lar, marked with close, rounded, radiating ridges from the beak, which is small, deflexed, and little more than 

 one-fourth the length from the anterior marcrin. 



This is the only species of Orbicula in the older rocks which is marked with radiating ridges ; the form 

 is somewhat irregularly conic, depressed, closely set with round, slightly irregular, radiating ridges. Length 

 four and a half lines, width three lines. 



Family ATHYRID^. 



The AthyridcB or Productas are distinguished as a family from all the other Brachiopods, by being defen- 

 sively armed with large, strong spines. 



That the Athyridce immediately follow the Delthyridce or Spirifers is, I tliink, evident, from the close 

 connexion which exists between the genus Orthis, among the Spirifers, and Leptagonia, M'Coy, in the pre- 

 sent family, so nearly resembhng each other that it is sometimes diflBcult to determine to wliich of them a spe- 

 cies may belong ; while the lunion on the other side to the Orbiculidce is evident from the similarity of their 



