BRAcHIOPODA. 813 
volution of the ribbon lies just within the margins of the valves, 
and the number of volutions is small. The spirals have their 
bases parallel to the lateral slopes of the pedicle-valve and their 
apices directed obliquely toward the center of the opposite valve. 
The jugum is a continuous band, variable in position and shape. It 
may originate on the posterior or anterior limb of the primary 
lamell, or be placed medially ; its apex is always angular and 
directed anteriorly and the lateral curves vary in length and 
degree according to their position with reference to the spirals. 
Type, Zygospira modesta, (Say) Hall. Hudson River group. 
Distribution. Lower — Upper Silurian. 
Subgenus Catazyga, Hall. 1893. 
(Plate 40. figs. 26-33.) 
Shells rather large, subcircular or ovoid, with valves more 
convex than in Zycosprra, 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 Zycosprra, is covered with a 
great number of fine radiating stria. The typical external expres- 
sion of Zycosprra 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 exca- 
vated, short, sharply defined and longitudinally striated impres- 
sion. In the brachial valve isa broad anterior and a narrow, 
elongate posterior pair of scars. The spirals are of essentially 
the same character as in Z. modesta, though ee 
the form of the cones is such that their apices 
converge toward the median line in a plane (iis 
just below the surface of the brachial] valve. 
The jugum, however, differs; it is, in the first 
place, persistently posterior in its position, 
originating as in Arrypa, the lateral lamelle oroaanoc ten an 
bending downward toward the bottom of the Fee obs hawing ile 
i : orm of the spiral cones 
brachial-valve and directed forward in lines ana jugum as viewed 
which are parallel for a short distance. Thence see oe Maat 
they bend inward and upward, meeting in a short angle in the 
space just behind the apices of the spirals. 
(Type, Catazyga -Headi, Billings (sp.). Lower and Middle 
Silurian.) 
65 
