INTRODUCTORY. 
TuE collections of Hydroids obtained by Mr. de Pourtalés during the ex- 
ploration of the Gulf Stream between Florida and Cuba, while assistant 
of the Coast Survey, were sent to me by Mr. Alexander Agassiz for deter- 
mination. The collection, which had been put up in spirits and is, for the 
most par, in an excellent state of preservation, proves to be a very large 
one, and to contain a great number of undescribed species. The determi- 
nation of the specimens, and the drawing and description of the new 
species, have occupied more time than I had anticipated, and with the 
pressure of other avocations have caused more delay than I could have 
wished in the publication of the results. 
All the enlarged drawings and details of structure have been carefully 
made by myself, while most of the drawings representing the natural size 
of the hydroid, as well as the completion of some of my sketches and the 
drawing of the whole on stone, have been executed by Mr. Hollick; and 
I must here bear testimony to the truthfulness of his work and the almost 
photographic actuality with which he has reproduced the natural form of 
the specimen. 
One of the most striking features of the collection consists in the large 
number of undescribed species, and the small percentage which can, with 
probability, be referred to forms known to exist on the European side 
of the Atlantic. 
Leaving out of consideration a few specimens whose characters, in con- 
sequence of their imperfect preservation, could not be ascertained, the 
collection consists of seventy-one species. Sixty-four are here figured and 
described for the first time, and none of these have as yet been known to 
occur beyond the area to which the exploration was confined. There thus 
remain only seven species which, so far as their identification is possible, 
