232 T. C. Brown — Fauna from Chappaquiddich Island. 



rower toward the ventral portion of the shell, while the pos- 

 tero- ventral margin is more nearly a straight line and the radii 

 are more numerous and finer and proportionately more widely 

 separated. In M. vineyardensis and M. plicatula the mode 

 of increase in the number of the radii by occasional intercala- 

 tion and bifurcation is very similar. In M. plieatula the 

 umbonal ridge is less angulate and less pronounced. 



The shell described is a left valve with the following meas- 

 urements : length 32*5 mm , width 14 mm . Several small specimens 

 of this same species occur in other fragments of the concre- 

 tions, as well as imprints of these shells. These smaller speci- 

 mens correspond exactly with the growth lines of the younger 

 stages in the larger individuals. 



Modiola vineyardensis mut. inornata. 



This mutation is very similar to the type of the species 

 described above, except that the radii are very faintly marked. 

 Fine, distinct, concentric growth lines mark the surface. Dis- 

 tinct radii can be seen on the anterior and umbonal region of 

 the shell. These radii are flattened on top and separated by 

 very narrow impressed lines. They fade out as they pass away 

 from the umbonal and completely disappear on the ventral 

 portion of the shell. 



This mutation is represented in the collection by a compara- 

 tively small left valve. 



Modiola Hollicki sp. n. PL VIII, fig. 2. 



Shell ventricose, with a prominent umbonal ridge extending 

 from the beak to the ventral margin ; shell sloping abruptly to 

 the anterior margin and becoming flat in the postero-dorsal 

 part ; anterior end rather short ; anterior end rounded, front 

 margin slightly arcuate, ventral margin broadly arcuate, 

 rounded, postero-dorsal margin obtusely angulate; cardinal 

 line straight ; surface without ornamentation except for rather 

 faint concentric lines of growth. 



In general outline this species somewhat resembles M. 

 Mitchelli Morris,* from the Eocene of England. It has a more 

 obtuse postero-dorsal angle and is slightly narrower, with a 

 slightly arcuate anterior margin instead of being emarginate 

 as in that form. 



Represented in the collection by two specimens, one nearly 

 perfect left valve (fig. 2) and a valve lacking the beak and 

 hinge area. These occur together with the Corbulas (see be- 

 low) in a fine-grained hard ferruginous lutyte concretion quite 

 different in character from the micaceous sandstone concretions 

 in which all the other fossils are found. 



* Palaeontological Society Monographs (see above), p. 68, pi. xiii, fig. 10. 



