66 HYDROZOA. GRAPTOLITOIDEA 



early forms the hydrothecse are very simple, differing 

 but little from the sicula, whereas in the later ones they 

 exhibit considerable modification. In some genera the 

 hydrothecse of different species show great variety of 

 form, those of one species being often much more like 

 those of a species belonging to another genus than to 

 other species of the same genus : thus we get the same 

 type of hydrotheca in the three forms Bryograptus cal- 

 lavei, Tetragraptus hicksi, and Didymograptus affinis, and 

 another type in Bryograptus retroflexus, Tetragraptus 

 denticidatus, and Didymograptus fasciculatus. It is con- 

 tended that each of these groups is a genealogical series 

 and should be regarded as a genus — that T. hicksi has 

 descended from B. callavei, and D. affinis from T. hicksi. 

 According to the old view all the species of Didymograptus 

 were thought to have descended from one common ancestor; 

 but this will not account for the close resemblance which 

 the hydrothecse of certain species of Didymograptus bear 

 to those of certain species of Tetragraptus] on the other 

 hand, this is readily explained if we consider that the 

 species of Didymograptus have descended from various 

 species of Tetragraptus. Then again, the remarkable 

 diversity in the hydrothecse of Monograptus can be easily 

 understood if we grant that the forms included under 

 this term are the descendants of different species of one 

 or more genera. But since species which have a different 

 ancestry cannot be placed in the same genus, we must 

 regard Monograptus as an assemblage of forms which 

 agree merely in consisting of a single uniserial branch or 

 stipe. 



It has been suggested that the similar modification in 

 branching found in different forms may have been brought 

 about by the necessity of obtaining sufficient food — the 



