Report of the State Geologist. 



523 



its structure. Atj&ypa, with its spirals directed upward into the 



cavity of the brachial valve, lias its jugum situated posteriorly 

 and is composed of two lateral branches directed anteriorly, and 

 slightly upward in the middle. In early growth-stages this is ;i 



Fig. 168. 



Brachidium of Glassia obovata, showing the posterior position of the loop. 



(Davidson.) 



continuous band, but in mature or senile conditions of growth it 

 becomes divided in the middle (probably from the lateral strains 

 to which it is subjected) and the extremities of the jugal processes 

 become much thickened. In Glassia the spirals have their apices 



Fig. 169.— Brachial valve of 

 M e g erlia truncata. 



g, loop: 1, septum. (Davidson.) 



Fig. 170.— TerebrateUa dorsata . n, foramen: o, deltidial 

 plates; c ', cardinal process ; /(.hinge-plate; /, septum; 

 g, brachial supports. (Sukss.) 



at the center of the internal cavity and their bases at the sides : 

 the jugum is a short posterior transverse band. In Zygospira 

 the variation in the position of the jugum in a given species 

 (e. g., Z. modesta) is remarkable, it being sometimes posterior, at 



75 



