132 Annals of thk Carnkuie Museum. 



Trigeria lepida Hall. 



Hall, Tal. N. V., VIII, Pt. 2, 1893, pi. 50, figs. 36-40. 



There are in the collection, about fifteen specimens of Trt\i;eria 

 lepida, three of which are adult individuals. The smallest specimen 

 is 1.09 mm. long and .93 mm. wide : the largest is 6.8 mm. long and 

 6 mm. wide. The largest specimen is smaller than the smallest one 



figured by Hall. That is 7.8 mm. long and 6.7 mm. 



wide, while the largest is 1 1 mm. long and 9. 5 mm. wide. 



According to that author, an adult has from 20 to 25 



plications. 



Description of Siuallcst Specimen. — The smallest 



individual is roughly triangular in outline, the rostrate 



beak, which projects .25 mm. beyond the hinge, form- 

 Hall; young ing the apex. The ventral valve is convex and smooth 

 individual, be- without folds on sinus. The delthyrium is narrow and 

 fore the incep- Qpgn. The dorsal valve is convex, nearly as deep as 

 !" ^ ^^^ the ventral, and has a deep, narrow median sinus. There 



tions. V16. ' . ' . 



are no striae. This valve is oval in outline, and wider 

 than long. The length is 84 mm., width .93 mm. 



A narrow form, only a little longer than this, has the following 

 dimensions. Length, ventral valve 1.36 mm., dorsal valve 1.20 mm., 

 width I mm. In this stage the form is very suggestive of CentroJiella 

 (Fig. 30- 



Chani:^es During Development. 



Outline. — The outline changes very little during development, 

 except in the amount of extention of the ventral beak. In the very 

 small specimens, less than 2 mm. in length, the length of the ventral 

 beak, behind the apex of the dorsal valve, is from .23 to .26 of the 

 length of the shell. In older stages the beak is much less prominent. 

 In a specimen 3.4 mm. long the beak is only . 19 of the length, at 5.5 

 mm. long it is .15 and on the largest specimen, 6.8 mm. long, it is 

 only .09 of the length (Fig. 32). 



There are, in all stages, both broad and narrow forms. In the 

 broad form, the index is about .85 in the young specin)ens and .90 

 in the adults. For the narrow forms it varies from .73 to .80. In 

 the adults of the broad form, the dorsal valve is about as long as wide, 

 and is only a little longer than wide in the narrow form. 



