PLASTICITY AND PERMANENCY OF CHARACTERS. 319 



two types continued on to the close of the Devonian, living 

 often together, but having an independent existence, and not 

 reaching a completely specific differentiation till the close. 



They are more properly claimed as varieties than as dis- 

 tinct species, this being chiefly due to the maintaining of 

 variability and the failure of disappearance of intermediate 

 forms linking the extreme and typical forms, which thus at 

 the beginning of the life-period of the genus quite fully ex- 

 pressed their characteristics. 



In the Closing Part of the Life-period of the Race the Extremes 

 of Acceleration and Retardation Expressed. In the last few 

 pages the characters of Atrypa have been described, and it was 

 pointed out that a certain part of the characters, those of 

 the species Atrypa reticularis and closely allied species, have 

 exhibited great persistence of variability. We observed that 

 this species, or race as we may call it, began at the opening 

 of the Silurian or possibly in the latter part of the Ordovician, 

 was conspicuous in the Silurian and the Devonian, but ap- 

 pears to have become extinct at the close of the Devonian. 

 At the close of the life-period of the genus the variability in 

 respect of rate and extent of bifurcation of the surface plica- 

 tions presents a tendency in two predominating directions. 



On the one hand, the bifurcation is rapid and extreme, and 

 the whole surface of the adult appears covered with numerous 

 fine plications : this would indicate rapid and continuous 

 bifurcation during growth, or the character of bifurcating of 

 the plications shows, in comparison with ancestors, accelera- 

 tion of development. 



On the other hand, there is a well-marked variety which 

 becomes sharply distinct from the others in the Neodevo- 

 nian and goes under another specific name, Atrypa spinosa, 

 which shows the opposite tendency ; the bifurcating has be- 

 come almost nil. The adult shows no more plications than 

 does the early stage of growth at the distance of one-fourth 

 inch from the beak: this is an expression of retardation of 

 this particular element in the growth of the shell. 



Summary. To define precisely those characters which are 

 considered in the above analysis, the following summary may 

 be given : In the geological series of forms described under 



