22 Mr. W. K. Brooks on the Origin of 



Fig. 3. Vertical section of the same, similarly enlarged. 



Fiy. 4, Vertical section of Bosenel/a clentata, Rosen, sp., enlarged twelve 



times. Silurian, Kattentack, Esthonia. 

 Fir/. 5. Tangential section of the same, similarly enlarged. 

 Fig, 6. Vertical section of RoseneUa imchyphytln, Nich., enlarged twelve 



times. Silurian, Kattentack, Esthonia. 

 Fig. 7. Tangential section of the same, similarly enlarged. 

 Fiy. 8, Vertical section of RoseneUa inacroeystis, Nich., enlarged twelve 



times. Wenlock Limestone, Gotland. 



Plate II. 



Fig. 1. Vertical section of Labechia ohioensis, Nich., enlarged twelve 



times. Ordovician (Hudson-Jliver Formation), Cape Smythe, 



Lake Huron. 

 Fig. 2, Tangential section of the same, similarly enlarged. 

 Fig. 3. Tangential section of Labeehia serotina, Nich., enlarged twehe 



times. Middle Devonian, Devonshire. 

 Fig. 4. Vertical section of the same, similarly enlarged. 

 Fig. 5. Vertical section of LabecJda canadensis., Nich. & Mur., enlarged 



twelve times. Ordovician (Jewesche Zone), Saak, Estlionia. 

 Fig. G. Part of an exfoliated specimen of Labechia ? Schmicltii, Nich., 



enlarged ahout six times, Silurian (Upper Oe.sel Zone), Hoheu- 



eichen, Oesel. 

 Fig. 7. Tangential section of the same, enlarged twelve times. 

 Fig, 8. Vertical section of the same, similarly enlarged. 



IV. — The Origin of Metagenesis among the Tlgdromedusce. 

 By W. K. Brooks *. 



Most of the recent writers upon the origin of the sexual 

 Medusre which are set free from communities of sessile 

 hydroidsj and upon the relation between them and the 

 hydrolds, agree in the opinion that the sessile community is 

 the primitive form, from which the Medusre have been derived 

 through division of labour, and the gradual specialization of 

 the reproductive members of a polymorphic hydroid cormus. 



In a monograph which has just been published in the 

 ^ Memoirs of tJie Boston Society of Natural History ' (" The 

 Life-history of the Hydro-Medusffi : a Discussion of the 

 Origin of the Medusa3, and of the Significance of Meta- 

 genesis ") I show that the life-history of the Narcomedusse 

 and Trachomedusge is irreconcilable with this view. The 

 accepted view regarding these groups of Medusae is that they 

 have been evolved from ancestors with a sessile hydra-stage 

 and an alternation of generations, and that they have gradu- 



* From the 'Johns Hopkins University Circulars,' No. 49, May 1886, 

 pp. 86-88. 



