BRACHIOPODA. 
155 
The crura are short and straight at their union with the primary lamellae, 
making a rectangular curve. The first half-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 
loop is a continuous band, variable in position and shape. It may originate on 
the posterior or anterior limb of the primary lamellae, 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, Producia modesta, (Say) Hall. Hudson River group. 
Observations. The existence of atrypiform spirals in Producia modesta {Atrypa 
modesta, Palaeontology of New York, Vol. I, p. 141), was recorded in the Thir¬ 
teenth Annual Report on the State Cabinet of Natural History, p. 69, and 
subsequently in the Fifteenth Report of that institution; the structure of the 
spirals and their connection was described and figured, and the new genus 
Zygospira, erected on the basis of these characters. 
Preparations of Zygospira modesta, (Say) Hall; showing the variation in position of the loop. (c.) 
