Grabau — Faunas of the Hamilton Group. 



277 



well marked and deep. Brachial valve convex, slightly ventricose below the 

 beak, regularly sloping to the base and lateral margins. "Width about one 

 and one-third times the height. Beak of brachial valve elevated above the 

 hinge line, which is less than the greatest width of the shell, and has its 

 extremities rounded. Area of brachial valve linear, that of pedicle-valve 

 high. In the center of the brachial valve there is usually a shallow depres- 

 sion, which becomes wider towards the front. Entire surface covered by 

 numerous elongated pits like those of A. spinosa, Clarke, and by concentric 

 striae, which become strong at intervals. On the brachial valve the pits have 

 a distinctly radial arrangement. On a well preserved specimen a part of the 

 surface shows short delicate spinules. 



Ambodoelia umbonata, var. nana, x 3. 



Fig. 3, Cardinal view, fig. 4, profile? fig. 5, ventral Tiewj fig. 6, dorsal viewt lig. 7. a portion of 

 the surface (x 10), showing elongate pits and the bases of the spinules which lay in them. 



This variety is characterized by its uniformly dwarfed size, its convex 

 brachial valve, and the strongly marked pits on its surface. The proportions 

 are likewise different from those of the ordinary forms of the species. 



Measurements of several specimens resulted as follows : 



Pedicle-valve: 12 3 



height ..... 6.3mm. 5.9mm. 5. mm. 



width - 7. mm. 6.6mm. 5.8mm 



depth 3.3mm. 3. mm. 2.5mm. 



Bracli ial valve : 12 3 



height ..... 5.5mm. 5. mm. 4.5mm 



width .... 7. mm. 6.6mm. 5 8mm. 



depth. 2. mm. 1.5mm. 1.2mm. 



