THE BRACHIOrODA 3 



anterior ; the length of the shell is from one to the other. 

 The breadth of the shell is measured at right angles to 

 the length, in the plane separating the two valves, while 

 the thickness is from the surface of one valve to that 

 of the other where they are farthest apart. 



FIG. 2. ORNITHELLA OBOVATA (J. SOWERBY). 



a, Dorsal view ; b, anterior view ; c, side view, all natural size ; d, umbonal 

 region of both valves, X f, showing cardinal area, foramen, and 

 deltidial plates in ventral valve, umbo and dark streak indicating 

 internal septum in dorsal valve. (Original.) 



The surface is nearly smooth, but at irregular intervals 

 it is marked by fine lines nearly parallel to the valve 

 margins, but becoming crowded together as they approach 

 the umbo. Evidently these lines, like the rings of a 

 tree or those of a ram's horn, mark stages of growth 

 (growth-lines) ; they must, in lact, represent the margins 



