Type species: Athyris headi Billings, 1862 (Jin et al., 1997)
Late Ordovician (late Caradoc – Ashgill)
Catazyga is an extinct genus of stationary epifaunal suspension feeders
Identification in Hand Sample:
- Shells small to medium, globose-elongate, biconvex-ventribiconvex
- Hinge short, narrow
- Pedicle opening obsucred, pedicle cavity is deep and in front of it lies a more deeply excavated impression
- Muscle scars strongly incised
- Jugum persistently posterior in its originating, bending inward and upward, meeting in a short angle in the space just behind the apices of the spirals
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- Small to medium, globose-elongate, biconvex-ventribiconvex; short, narrow hinge; small anacline to hypercline area; obscured pedicle opening; minute deltidial plates; finely ribbed; rarely fine concentric growth filae; weakly sulcate-rectimarginate to weakly plicate commissure; thick pedicle callist; muscle scars strongly incised; teeth stubby, curved, with or without dental cavity; hinge plate massive; bulbous crural bases; crura short; mediodorsal spiralia with fewer than 10 whorls; simple, posteromedial, U-shaped jugum. Distinguished from Anazyga by large size, thick shell wall, nature of hinge plate, posterior location of jugum.
- Shells rather large, subcircular, or ovoid with valves more convex than in Zygospira, the rotundity of the pedicle valve obscuring the usual prominence of the umbo in that genus. Both valves bear a low median sinus, while the external surface, instead of being coarsely plicated as in Zygospira, is covered with a great number of fine radiating striae. The typical external expression of Zygospira is thus to a large degree lost. On the interior of the pedicle valve the muscular impressions are well defined; the pedicle cavity is deep and in front of it lies a more deeply excavated, short, sharply defined and longitudinally striated impression. In the brachial valve is a broad anterior and a narrow, elongate posterior pair of scars. The spirals are of essentially the same character as in Z.modesta, the form of the cones is such that their converge toward the median line in a just below the surface of the brachial valve. The jugum, however, differs; it is, in the first place, persistently posterior in its originating, as in Atrypa the lateral bending downward toward the bottom of brachial valve and directed forward in which are parallel for a short distance. Thence they bend inward and upward, meeting in a short angle in the space just behind the apices of the spirals.
- Diagnosis: Small to moderatly sized, elongate, finely ribbed, biconvex to ventribiconvex, weakly folded zygospyrids with anacline-hypercline beaks covering a minute pedicle opening and deltidal plates (normally not visible). Internally, the pedicle cavity and muscle field is deeply incised and contains thick irregular calcite deposits; small teeth in socket cavities have a massive centrally directed base, dental cavities absent or elongated horizontally. Ventral adductor muscle field broad, subrectaogular, divided by weak median ridge; diductor area narrow, tapering posteriorly, poorly defined ; dorsal adductors elongate, narrow. Dorsal valve with small cardinal process on thick hinge plates, strong socket plates support small crura. Posteriorly located jugum bent towards cone apices; spiralia three to ten whorls.