Report of the State Geolo* 



51 



The size of the spiral cones and their direction varies to a cer- 

 tain degree equably with the form of the interior cavity of the 

 valves. In species of Spimfer with greatly extended hinge-line 



the cones are very Long and arc directed toward tin 1 cardinal 

 angles; where the hinge is short I Martinia) and its lateral angles 

 rounded the cones are short or placed at oblique angles. In 

 Mkrista, Athyris, Rhynchospira, Mkristixa, etc., etc., the axis of 

 the spirals lies in the greatest transverse diameter pf the shell. 

 In Atrypa the two flattened cones lie side by side, and their 

 apices are directed into the cavity of the deep brachial valve. 



Fig. 148.— The brachial supports of MageJlania australis. (Hancock, j 



The brachial supports are attached to the hinge-plate, or apical 

 portion of the brachial valve, by two processes termed crura. 

 The precise termination of the crura and the commencement of 

 the spiral ribbon is often obscure, but is frequently indicated by 

 a sharp angle at the point of union and an abrupt widening of 

 the ribbon directly behind it. (Nucleospira, Rhynchospira.) 

 From this the curvature of the ribbon is downward into the 

 cavity of the brachial valve, and the extensions of the ribbon for 

 the first one-half revolution are termed the primary la/mellaB. 



In Spirifer the crura are very long, the bases of the spiral cones 

 in this genus being small and situated well forward. The pri- 



69 



