118 



THE AMERICAN NATURALIST 



[Vol. XL 



the shell, plotting the angles and lengths of the successive growth 

 lines in crossing them (Fig. 30). Thus when the shell surface 

 becomes smooth in old age the zigzag lines of growth where pres- 

 ent represent the successive positions of the mantle border. It 

 is as if the plications had been merely transferred from the vertical 

 to the horizontal plane, as the actual 

 mantle is probably scalloped to the 

 same degree in both cases and the 

 absence of ribs results simply from the 

 changed angle of curvature of the shell. 

 Often, hovever, there is a tendency of 

 the mantle edge to fill out the scallops 

 and to present a smooth edge. A 

 beginning in this direction can be seen 

 at the cardinal angles of many plicate 

 individuals. Examples of this are noted 

 in very old specimens of Spirifer oweni, 

 Rhynchotrema capax, etc. 



The continued anterior growth after 

 the practical cessation of lateral growth 

 causes the cardinal angles to increase 

 orthl^pieSdinrsup™^^^^^ s^^^ causes also the shell index 



1 one anoTher.yowing graph- to decrease (scc sections 6 and 7). 

 furrL^I" pHiHcatfonr^ 'simis This is a taking-on again of the large 

 hner'^°TheiXures^were^^^^ Cardinal angles and small index of the 



ted from the median sinus of a nepionic Stage. 



cSai^No. ill, H^\ZT^x2i. ^'ot ouly is there repetition of youth- 

 ful characters in the outline of the shell 

 but there is also a similar repetition in the loss of ornamentation, 

 for the nepionic shell is smooth. An old man with his bald head, 

 curved back, toothless gums, and size smaller than during matu- 

 rity, resembles the child. Though in these and in many other 

 respects the resemblance is very striking yet in the child the form 

 is the result of positive, developing factors; in the man it is nega- 

 tive, degradation^ (see also Hyatt, '97, p. 218). So among 

 brachiopods the enlargement of the cartlinal angles, reduction 

 of shell index, and the obliteration of ribs, spines, nodes, etc., are 

 in a certain sense a return to the features seen in the nepionic 



