On a New Exogyra from Del Rio Clay 21 



of growth begin to develop, interrupting the ribs and im- 

 parting to them the aspect of roof tiles. 



The umbonal ridge is as prominent as in the preceding 

 stage at least in the umbonal region, but toward the post- 

 ero-inferior region it becomes rounded and less distinct. 



The shape of the shell begins to change. It has no longer 

 the triangular form but begins to coil spirally. In connec- 

 tion with this circumstance the growth lines and lamellae 

 no longer form a perfect elliptical curve, but begin to show 

 in their course a distinct protuberance or tongue-like pro- 

 longation on the umbonal ridge. 



The next stage represents the beginning of the mature 

 development (PI. IV, fig. 10-15; PI. V, fig. 1-3). The ribs 

 disappear about as suddenly as they began, the shell becomes 

 smooth and is only covered by lines of growth and lamellae 

 which appear more distinct the more the shell grows. They 

 no longer form a simple curve, but are rather wavy, especial- 

 ly on the posterior side and a little less on the anterior side, 

 showing a pronounced tongue-like prolongation or protu- 

 berance in their outline on the umbonal ridge. The wavy 

 appearance of the lamellae on both sides corresponds to the 

 development of shallow furrows (PI. V, fig. 4, 17), one more 

 distinct on the anterior side and one generally less so on 

 the posterior side. 



The shape of the shell now changes very rapidly, .coiling 

 spirally so that the umbonal region becomes twisted and de- 

 tached from the rest of the valve. 



Exceptionally it can be observed that very faint and some- 

 what irregular longitudinal ribs appear again on larger 

 shells up to a distance of about 35 mm. from the beak, 

 measured along the umbonal ridge (PI. V, fig. 17). 



In the last stage of mature specimens the shell attains its 

 characteristic shape similar to a ramshorn, as described by 

 Roemer (PI. IV, fig. 1-9, 16-18; PI. V, 4-6, 17). 



The opercular upper shell does not change essentially 

 (PI. IV, fig. 4; PI. V, fig. 16, 18-23). It appears to have 

 the same character from the size of about 2 mm. to the 

 largest mature forms with a length of 25 mm. and more. The 

 valve is sometimes nearly flat, always with a rather deep 



