REPORT OF THE DIRECTOR I916 127 



catenulatus which appears in the Ordovician, flourishes in the 

 Silurian and still persists into the lower Devonian, may be cited 

 here. Some of these, as Halysites, belong under the heading of 

 persistent sessile forms; the others, as Atrypa reticularis and Lep- 

 taena rhomb oidalis, are clearly forms of extreme robustness and 

 fertility but slight variability. 



That the persistence of these species is but more or less relative 

 was interestingly brought out in a discussion at the New Haven 

 meeting (1912) of the Paleontological Society, when Prof. H. S. 

 Williams mentioned Leptaena rhomb oidalis as a form that is per- 

 sistent because it is a form of great plasticity and very variable in 

 every locality and horizon where it occurs; therefore, he believes 

 it has not been separated although its differences are as great as 

 those of several genera, like Leptostrophia and Stropheodonta. 

 Dr E. O. Ulrich asserted that it is not so variable in each place, 

 there being but little variation of individuals, but that it is made up 

 of a multitude of species of different horizons that can be distin- 

 guished. Dr A. F. Foerste has, indeed, distinguished some of these 

 species. The fact that Leptaena rhomb oidalis has so long been left 

 undivided by paleontologists who are ever ready to distinguish 

 species would still indicate that it represents a long-existing compact 

 group of little changing forms of a relatively persistent type. 



Persistence in Higher Groups 



There are also a number of families and orders that are remark- 

 able for their wonderful persistence without material change. 

 Such are the limulids among the Merostomes, which begin in the 

 Devonian; the scorpions, arising in the Silurian and culminating 

 in the Carboniferous, but still in vigorous existence today with but 

 little change; of the Carboniferous Eoscorpius it is stated (Z. E. 

 p. 788) that it does not differ in any important respect from living 

 forms; the Pedipalpidae, known in a considerable number of types 

 from the Carboniferous and but little different today; the cock- 

 roaches which, with 300 species from the Paleozoic of North 

 America alone, were then the prevailing type of insects and still 

 flourish but little changed, the true Blattinidae beginning in the 

 Mesozoic; various families of the bryozoans, especially of the 

 Cyclostomata ; the lingulids etc. 



The causes of persistence here applied to the genera also hold 

 true in a more generalized form, for the larger groups. In some 

 cases, as in the scorpions, a superior set of offensive and defensive 

 arms (the pincers and poison glands), early developed, have 



