20 University of Texas Bulletin 



Roemer remarks that the surface of the lower valve in 

 the region of the beak is covered by delicate longitudinal 

 striae interrupted by the lamellae of growth and which gen- 

 erally can be seen only in the more juvenile specimens, the 

 region of the beak being more or less corroded in the larger 

 individuals. 



A study of well preserved specimens of Ex. arietina of 

 different ages shows at once that Roemer is absolutely cor- 

 rect in his description, but also that the development of the 

 species shows three clearly marked stages. 



Leaving aside the protoconch which is of the size of about 

 a common pinhead, the first stage of development (PI. V, 

 fig. 7-10) has the following character: In shells up to 

 about 3 mm. in length, the surface does not show a trace of 

 ribs, even where not the slightest corrosion has taken place ; 

 it is only covered by rounded, concentric, relatively broad 

 and not very distinct folds and the growth lines. At this 

 size, the posterior side of the shell is distinctly flat with the 

 beak pressed against the rest of the valve; only the very 

 first part of the beak in a length of 0.5 to 1.0 mm. elevates 

 itself a little above the surface of the shell. The umbonal 

 ridge is relatively sharp and high and reaches from the 

 beak to the postero-inferior margin. The anterior side 

 forms an angle of about 50 degrees with the posterior side, 

 and is rounded in height as well as in width. This angle 

 between the two sides causes the prominent umbonal ridge. 

 The outline of the valve is triangular-suboval at this stage. 



The second stage is represented by specimens of a length 

 of about 10 mm. or a little more (PI. V, fig. 11-16). In 

 these individuals the shell of the first part of the beak, up 

 to about 3 mm. from its point, is of course smooth, but on 

 the next wrinkles of growth, fine radial ribs appear sud- 

 denly in the number of about 30 to 35. They are very dis- 

 tinct on the anterior side, the umbonal ridge, and the greater 

 part of the posterior side, but become indistinct in this lat- 

 ter region on the surface near the end of the beak. The 

 ribs reach higher on the anterior than on the posterior side, 

 and pass obliquely over the umbonal ridge. The lamellae 



