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PALAEONTOLOGY OF NEW-YORK. 
©EMUS AthYRIS (McCoy). 
Athyris , McCoy, 1844. Spirigera , D’Orbigny, 1847. 
The Genus Athyris was established in 1844 by Prof. McCoy, upon 
certain species separated from the Terebratul,® ; and when restricted 
according to the original types of that author, includes a very natural 
group of shells, but which nevertheless possess many external features 
in common with the later established genera Merista and Meristella, 
and from which the species are distinguished by important internal 
characters. 
The shells of the genus are variable in form, being suborbicular, 
transverse or elongate, subglobose or depressed, and-sometimes suban- 
gular. The typical species are depressed suborbicular. The structure of 
the shell is fibrous; the surface in most species is strongly marked by 
concentric striae, and some of them are strongly lamellose, with the 
lamellae becoming fimbriate or pectinate. In this aspect, as well as in 
general form, these shells have a resemblance to some of the Spirifers 
with short area and rounded cardinal extremities. The surfaces are 
sometimes indistinctly radiatingly striated; but these striae are usually 
subordinate to the concentric striae or lines of growth; and it may be 
doubtful whether any well authenticated species of the genus has con¬ 
spicuous radiating striae or costae. 
The apex of the ventral valve is usually or perhaps always perforated 
by a rounded foramen, tbe lower side of which is formed by the umbo of 
the opposite valve. When the valves are separated, this foramen commu* 
nicates with a triangular space, which opens into the main cavity of the 
valve. This triangular fissure, which in older shells is usually occupied 
by the beak of the opposite valve, has at some time during the animal’s 
life been closed by deltidial plates. 
The dorsal valve is furnished with a strong cardinal process, the 
centre of which is often depressed and spoon-shaped, but sometimes thick- 
