326 STUDIES IN EVOLUTION 



Development of Leptcena rhomboidalis. — Continued. 



Among the mature shells the greatest variation is to be 

 found in the development of the anterior curtain, or genicu- 

 late and sloping marginal area of the valves. In some speci- 

 mens this is so excessively developed that the posterior or 

 concentrically undulated portion of the ventral valve is at 

 right angles to the plane of the margin. Also, in many 

 specimens the curtain is obscurely plicate, and the radiating 

 striae are often irregular and sometimes fasciculate, while on 

 the upper part of the valves these strise are very uniform 

 in their arrangement. No specimens have been noticed 

 which are so strongly quadriplicate as those illustrated by 

 Mr. Davidson, on Plate XXXIX of the "British Silurian 

 Brachiopoda." 



Senile specimens usually have the valves very much thick- 

 ened from internal growth, and the margins show strong 

 varices. It is noticeable that nearly all the old shells are 

 covered with a growth of Cranias, Bryozoa, Favosites, etc., 

 and it is very difficult to free the shell from this overgrowth. 

 In consequence of this, many of the shells are scarcely recog- 

 nizable, and resemble agglomerations of Bryozoa and corals. 



