INTRODUCTION. 5 



mud, or attached to rocks. Under either condition the power to 

 grow rapidly in height is essential to existence. Otherwise those 

 animals that adopt the former mode of life would be buried by the 

 accumulation of silt, while those that adopt the latter would be 

 choked by the growth of sea- weed, or be starved owing to the 

 capture of all the food by taller rivals. 



To the zoologist, who can base his classification upon the soft parts 

 of the animal, as well as upon the hard, these tubicolar forms present 

 no exceptional difficulties. They are, indeed, of especial interest, 

 for they show how close a resemblance may exist between animals 

 of different groups, owing to adaptation to suit the same conditions 

 of life ; and they supply the most emphatic warning that similarity 

 in structure does not always imply community of origin. 



To the paleontologist, however, who cannot check his con- 

 clusions by the evidence of vascular anatomy or embryology, these 

 tube-dwelling animals are a vexation and a puzzle. There is so 

 frequently no correlation between the structure of the tube and 

 of the creature that made it. Hence the vast majority of the 

 fossil worm tubes are now ignored by palaeontologists ; their study 

 has been abandoned in despair, for, so far as we can see at present, 

 they can give no aid in the classification of the group to which 

 they belong, or in tracing its life history. 



In the study of the Jurassic Bryozoa we are faced by the initial 

 difficulty that their shells are tubular. In periods earlier than the 

 Jurassic, many of the Bryozoa belonged to the order Cryptostomata ; 

 and in later times the order Cheilostomata was well represented. 

 In these two groups the skeletons of the individual members of the 

 colony are complex, and offer fairly reliable diagnostic characters. 

 But in Jurassic times the former order was extinct, and the latter 

 was represented by only two rare species; 98*5 per cent, of 

 the forms have left no traces, except their tubular skeletons. 

 Nevertheless, these fossils cannot be ignored like the worms, for, 

 with the two exceptions noted, they belong to orders either wholly 

 extinct or now of dwindling importance. Their evidence, therefore, 

 must be considered in any attempt to trace the evolution of the 

 Bryozoa. 



As tubicolar skeletons occur in so many different groups of 

 animals, in the study of the fossil Bryozoa we are faced at the 

 outset by the problem, how to recognize the fossil members of this 

 class. It must be at once admitted, that there are no diagnostic I 



