34 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 1 1 , NO. 2 



of the shell around the hole. Along the overlapped margin the cos- 

 tae are arranged in a cross-hatched design. It is possible that the 

 original break was much smaller than the lobe of reversed sculpture 

 and it may possibly have been subcircular in outline. Along a por- 

 tion of the inner and lateral borders the "patch" appears to have 

 overlapped the edges of the break and to have failed to meet the 

 shell smoothly where a gaping crack in the mold appears to denote 

 an irregular ridge which joined the two. 



2. Origin of abnormal lobe by regeneration. — The irregularity of the 

 sculpture of the "patch" and its lack of resemblance to that of any 

 of the associated organisms exclude the possibility of its being merely 

 a portion of another shell pressed against the shell of the derbya or 

 adhering to it. It seems most certainly to be an instance of regen- 

 eration, or replacement of a broken portion of the shell. The mantle 

 must also have been damaged by the breaking away of the shell and 

 this injury probably supplied the stimulus which brought about the 

 regenerative growth. The new growth was thus partly a pathologic 

 phenomenon. 



3. Origin of the injury to the shell. — How such a break in the shell 

 may have been caused is not altogether clear. The absence of peb- 

 bles and coarse sand in the sediment which enclosed the shell de- 

 notes comparatively quiet waters and eliminates the likelihood of its 

 having been caused by the pounding of shells, or shells and stones, 

 by wave action. The break appears to have been irregular in outline, 

 and unlike the neat round holes drilled in modern shells by boring 

 snails. It is possible that an enemy among the fish was responsible 

 for the injury. 



4. Mode of regenerative shell growth. — From the inward divergence 

 of the costae of the regenerated shell it seems likely that the new shell 

 material grew in the direction of divergence across the break and that 

 the growth did not begin simultaneously all over the damaged area. 

 Increase in extent in the normal shell is in a forward direction, and it 

 is probable that, as in the case of the Pelecypoda, as the thickened 

 margin of the mantle is advanced by mantle growth it deposits the 

 larger bulk of the shell material. In the case of the shell which we 

 are studying, it would appear that the direction of growth was re- 

 versed. Since we have no way of knowing the extent to which the 

 mantle was damaged we cannot determine the method by which it 

 replaced the shell. It is conceivable that the thickened margin may 



