22 MEMOIRS OF THE STATE MUSEUM 



Among the mature shells, the greatest variation is to be found in 

 the development of the anterior curtain, or geniculate and sloping 

 marginal area of the valves. In some specimens 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 striae are very uniforrn in their arrangement. No speci- 

 mens have been noticed which are so strongly quadriplicate as those illus- 

 trated by Mr. DAVIDSON, on plate 39 of the " British Silurian Brachiopoda." 



Senile specimens usually have the valves very much thickened 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 

 recognizable, and resemble agglomerations of bryozoa and corals. The 

 only other species of brachiopoda at this locality commonly thus over- 

 grown and involved, is Atrypa reticularis. 



StropJiomena rhomboidalis is cosmopolitan and has been discussed by 

 many authors who have shown its great variation and wide distribution. 

 So far as known, the youngest specimen heretofore figured is one repre- 

 sented by Mr. DAVIDSON.* This is an individual belonging to the third 

 stage of development, having a length of nearly 6 mm. and a distinct 

 circular perforation of the beak. 



* British Fossil Brachiopoda, Vol. Ill, Devonian and Silurian, pp. 283, 284, pi. 30, fig. 6. 

 The same. General Summary to the British Fossil Brachiopoda, p. 289. 



