476 



Forty- fifth Report on the State Museum. 



Ornamentation of the External Surface. 

 The surface of the brachiopod shell is rarely entirely smooth. 

 It may bear only the concentric lines or varices of growth, and 

 sometimes squamous, lamellar expansions, but as a general rule, 

 the surface is covered with radiating striae, lines, plications or 

 ribs, which may either be of equal size, may alternate in size, or 



CJL2RJS1 



Fig. 40.— The lamellose surface of Atrypa 

 aspera. 



Fig. 41.— The concentric lamellae of Atrypa 

 imbricata; enlarged. (Davidson.) 



be arranged in fascicles. Such configuration is of secondary 

 growth, the incipient shell being free from it, except in rare 

 instances where evincing an acceleration of development in this 





Fig. 42 —A portion of the exterior of Atrypa Fig. 43.— The surface spines of Atrypa hystrix. 

 reticularis; showing the extension of the 

 lamellae and their tendency to fold upward 

 into hollow spines. 



respect, or in other words, an earlier inheritance of specific 

 characters. In the plicated shells, the plications of one valve 

 generally alternate in position with those of the other, so that 

 on the margins, the edges of the two series arc interlocked, 

 effecting complete closure. 



28 



