INTRODUCTION. 2 1 



zooecia and low peristomes arranged on long, curved, regular lines. 

 Dimtopora michelini (Blv.) is similarly analogous to Berenicea 

 concatenates, Reuss. 



It may be urged that if this parallel series proves anything, it 

 proves the uselessness of the retention of the divisions Berenicea 

 and Dimtopora ; and it certainly does suggest that an interchange 

 of form may he produced by a sudden change of environment. 

 But in other periods the erect and encrusting forms both lived 

 together, and then frondose individuals probably gave birth to 

 frondose young, and adnate individuals to adnate young. Hence 

 in these cases the distinctions probably obtained for an indefinite 

 period, and to ignore the great zoarial difference in this case 

 would be a greater mistake than to accept it in others. Zoarial 

 characters are the only ones available for systematic work among 

 the Cyclostomata ; and, as the differences appear to hold as long 

 as the conditions remain approximately the same, I feel bound 

 to accept the divisions based upon them. These examples, 

 however, show that variations in this group are of very different 

 value from those on which genera are based, in some other groups 

 of Invertebrates. 



Diastopora and Berenicea have been taken as illustrating the 

 value of zoarial characters in the Cyclostomata, and they seem to 

 prove two things 



1st. That under similar conditions there is a tendency for suc- 

 cessive generations to have the same habit of growth. 



2nd. That a sudden change in environment may lead to a 

 sudden change in zoarial habit. 



In consequence, alternative courses are offered us in respect to 

 the treatment of zoarial characters. They may be ignored, and 

 forms placed by most authors in different families may be included 

 in one genus; or we must admit that there are no true genera 

 among Cyclostomata, but only certain convenient, but artificial, 

 groups of species. 



The latter alternative is the one to which I have been 

 driven, I must confess, rather reluctantly; for it means that 

 there is no hope in this order of ever establishing divisions 

 with the same absolute diagnoses as in most other groups of 

 Invertebrates. 



