388 C. E. Beecher — Revision of the Families of 



Conclusions. 



It is impossible, with present knowledge, to go deeply into the 

 chronological history of the genera of the Terebratellidse. They 

 certainly appeared before Jurassic time, because they w^ere then 

 well represented by several characteristic genera, viz : Kingena, 

 Ismenia, Zellania, and Megathyris^ although some of them may 

 represent incomplete developments of higher forms. Also, as 

 among recent species, several separate generic and specific names 

 may have been given to stages of growth of a few species. It is 

 evident that, in the identification of specimens in this familj^, 

 whether recent or fossil, the strict specific characters must be given 

 first consideration. Species, therefore, must be based upon surface 

 ornaments, form, and color, within certain limits, and genera <'nly 

 upon structural features developed through a definite series of 

 changes, the results of which are permanent in individuals evidently 

 fully adult. 



The austral distribution of the Magellaniinm and the boreal dis- 

 tribution of the Dallininm have been emphasized already, but, as 

 has been stated, this difference in geographical position in itself 

 does not necessarily constitute a basis for subfamily" separation. 

 The facts of development, however, and the distribution of the 

 genera show that the radical stock was dispersed probably in miso- 

 zoic time, and since then evolution has gone forward through differ- 

 ent lines of progression, and has terminated in similar types of 

 structure. 



In each line of progression, the acceleration of the period of 

 reproduction, by the influence of environment, threw off genera 

 which did not go through the complete series of metamorphoses, but 

 are otherwise fully adult, and even may show^ reversional tendencies 

 due to old age; so that nearly every stage passed through by the 

 higher genera has a fixed representative in a lower genus. More- 

 over, the lower genera are not merely equivalent to or in exact par- 

 allelism with the early stages of the higher, but they express a per- 

 manent type of structure, as far as these genera are concerned, and 

 after reaching maturity do not show a tendency to attain higher 

 phases of development, but thicken the shell and cardinal process, 

 absorb the deltidial plates, and exhibit all the evidences of senility 

 and reversion presented during the old age of the higher genera. 

 Krausslna shows partial loss of deltidial plates and a thickened 

 septum, with two strong prongs representing the ascending branches, 



