77/r Life-Histurii of Siiirifcr IwrLs. 147 



entirely wanting radiating plications. Tlie manner of disap})ear- 

 ance is worthy of notice. The undulations of the surface, form- 

 ing the plications, become lower and lower as the plications be- 

 come obscure, bat never in this way do they reach obliteration. 

 AVe find them obliterated first in the part of the shell represent- 

 ing the earlier stage of growth — about the beak; this (unpli- 

 cated) area becomes greater and greater until the plications are 

 confined to the margins, and they are obscure and' faint, — as in 

 the rare specimens of Imvis. 



The formation of folds or plications is thus seen to be a pro- 

 cess which, in the series of individuals under consideration, Ijegins 

 later and later in the growth of the shell, and in the last indi- 

 vidual has scarcely begun when matnrit}^ is reached, so that 

 only the margins are affected, — leaving the main part of the 

 surface free from i)lication. 



The facts that only the largest individuals ot Sp infer loevis, — 

 those whose growth continued the longest, — shoAv any trace of 

 the plications, and that whenever they are found, it is only on 

 the mai-gins of these large shells, are quite consistent with the 

 supposition that IcBvis is traceable genetically to S. fimbriatus of 

 the preceding period. 



In regard to the seventh character {g), let us first read what 

 Uall says of it in 6*. fimhriatus. 



'' The concentric stria? are studded with elongated nodes or 

 tnbercles, which are thus arranged in parallel bands, more or 

 less contiguous, according to the distance of the concentric 

 . strias. 



'•The elongate tubercles may perhaps more jjroperly be re- 

 garded as interrupted radiating striae, which, in the perfect 

 condition of the shell, have doubtless extended in slender spines 

 or sette. (They are termed by Mr. Conrad short longitudinal 

 stria^.)" 



These "short longitudinal stria?" are very characteristic of 

 S. fnidriatns, and much coarser and stronger than in any spe- 

 cimens of S. Icevis, but showing a tendency to become finer and 

 less strong in the smoother unplicated varieties of the former 

 species. 



