78 



TRANSACTIONS OF THE 



[JAN. 11, 



The adaptation involves the whole sui'face with the exception 

 of a small area (between the lines /landZ), near the umbo, which 

 is subdivided by two prominent growth lines into three portions. 



Fig. 1. Left valve of Saxidomns sqtialidus ? with shell of PUicuanomia 

 »nacrosc/(isma attached near anterior margin. The broken lines, a -I, etc., in- 

 dicate concentric strise of the Saxidoimis, the corresponding full lines those 

 assumed by the Placnanomia, and the small striated area between the lines h 

 and I, the umbonal region of the latter. 



The two outer subdivisions have respectively two sets of fine 

 longitudinal lines that are not quite continuous with each other 

 nor with those of the rest of the shell. These I believe to be mark- 

 ings assumed from a surface of attachment during earlier periods 

 in the life of the shell; their discontinuity at the lines of growth 

 evidently indicates that the Placuanomia must have twice shifted 

 its position. This probably took place from a position lower down 

 on the Saxidomus where the lines are finer, or possibly from an- 

 other shell. The possibility at first suggested itself that these two 

 sets of lines on the two outer umbonal regions might be radial 

 markings proper to the shell in its 3^oung stages. Such abrupt 

 change in surface markings is very common among the fossil 

 Brachiopoda and Cardiida? and has been observed by me in cer- 

 tain Gastropods. In Placuanomia, however, these are sharp, 

 few in number and large, relative to the shell, and are separated 

 by intervals that correspond very nearh' with the distances be- 



