PLATE :^0. 



Page 



Fig. 1 a - e, &, 2 a - r. RHrNCHONELLA mutabilis. 226 



1 a- e. Ovoid forms of this species. 



2 a, b, c. Profile views of the more gibbous forms, sliowiiig tlie gradual expansion of tlie 



dorsal valve. 



2 d, e. Front and dorsal views of a very gibbons form. 



2/-p, and «. Illustrations of extremely gibbous forms, which sometimes present a sub- 

 quadrangular outline, and some variation in the form of the broad extension of the mesial 

 sinus in front, and in the proportional length and breadth of the dorsal valve as shown in 

 2 I and 2 o. The form 2 n is rare, but is nevertheless one which the species sometimes 

 assumes. 



2 r. Cardinal view of a cast of this species. 



