Fig. 31. 



? Corymorpha pe i 



101 



cumference, if O denotes that no tentacle has been 

 developed in the prolongation of the chymiferous 

 tube : 



2 1 — Ti = 1 < ; or O + O + Ti + O, for the first 



set. 



2t = Ti+2T2 = 3<; orO + T2 + Ti+ T^ 

 for the second set. 



2 t = T, + 2 T^ + Tg = 4 <; or T3 + T^ + Ti 

 + T2 (fig. 31) * for the third set. 



Figures 12, 15, 19, 24, 30, have been lent to 

 me by my father, from the wood-cuts of the fourth 

 vol. of his Contributions to the Natural History of 

 the United States. Fig. 31 has been copied from a 

 drawing lent to me by Prof. H. J. Clark. The other 

 figures are copied from drawings made by me during the last two 

 years at Nahant, Beverly, and Naushon. It was not till the appear- 

 ance of the fourth volume of the Contributions of Prof. Agassiz that 

 it became possible to trace the intermediate stages of growth of 

 the many Hydroids of our coast, the relations of which to our free 

 Medusae had been traced during the investigations necessary for its 

 publication. 



On the subject of the present paper, the above-mentioned work 

 contains only a few facts relating to Tiaropsis, and a general infer- 

 ence, derived from isolated facts, that the distinction of species, based 

 upon the number and arrangement of the tentacles, can no longer be 

 considered valid. See, for instance, the modifications which have 

 been proposed in the tabular view of the Hydroids, in Vol. IV. of 

 the Contributions to the Natural History of the United States, with 

 reference to the numerous species of Forbes and of Gegenbaur. 



Professor Agassiz mentioned that the Museum at Cambridge had 

 recently received a cast of the great Megatherium of the British 

 Museum. This cast had been sent by Joshua Bates, Esq., of London, 

 to whose munificence the Museum was indebted for this valuable ad- 

 dition to their collections. He had taken this opportunity of compar- 

 ing the bones of the South American species with some fragments of 

 Megatherium bones which he found several years ago near Savannah, 



* Owing' to the great difficulty of distinguishing the free Medusae of Corymor- 

 pha (which has only been discovered this spring) and tha.t of Hylocodon, I have 

 marked the Medusa of the former as doubtful. 



