BRACHIOPODA. 



155 



The crura are short and straight at their imion 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, Produda modesta, (Say) Hall. Hudson River group. 



Observations. The existence of atrypiform spirals in Produda 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. 



Fig. 146. 



FIG. 147. 



Fig. lU. FIG. 149. 



Piep.arations of Zygospiramodesta, (Say) Hall; showing the variation in position of tlie loop. 



(C.) 



