12 SURFACE FAUNA OF THE GULF STREAM. 
every respect with the float of Velella or of Porpita, with broadly open ram- 
ifications communicating with the base of the single large central polypite, 
and differmg from it only in being fixed. We can readily imagine a slightly 
more advanced stage, with the additional proliferous polypites, or others, 
developed at the base of the central polypite and setting the attached disk 
free, we have to all appearances a somewhat modified Porpita or Velella. 
But Porpita seems to be also allied to another group of Hydroids, with 
which thus far no attempt has been made to compare them. I mean the 
Hydrocorallinss. My basis for this comparison rests upon the presence of 
the smgular white plate of Kolliker, and of its peculiar structure, — which 
reminds us of the porous structure of the corallum of Sporadopora, Allopora, 
Millepora, etec., although, of course, not having the regular horizontal floors 
of the latter, yet possessing, like these genera, large pits, the whole mass 
being riddled with passages and openings, forming the spongy mass of the 
white plate. Although some of the proliferous polypites of Porpita appear to 
rise from the larger of these pits, yet the others do not seem to hold any 
definite relation to them, beyond the fact that these proliferous polypites 
are limited to the ring occupied by this white plate. If this homology is 
correct, it shows how far-reaching are the affinities of the Porpitidse,— on 
the one side recalling from the structure of their white plate the corallum 
of Milleporidee, which date back to the cretaceous period; and, on the other, 
the similar structure of the Helioporidz, which, as is well known, have been 
shown by Moseley to be Haleyonoids. Whether the Stromatopore have any 
relationship to either of these groups or are sponges cannot at present be 
determined; but should they be related to the Milleporidx, the peculiar 
structure of the corallum of the Milleporidee, Stylasteridxe, and Porpitida 
would date back to the earliest Silurian. It is interesting to speculate, if 
the affinity of Porpita is greater to the Milleporide with a porous fixed 
corallum, or to the Tubularians having only a chitinous fixed basis. 
Porpita Linnzana Liss. 
K6lliker * was the first to give a detailed description of the Mediterranean 
Porpita. The Florida species is closely allied to it. It differs from it in size, 
the largest of our specimens measuring no less than 14” in diameter, while 
the medium size of the P. Mediterraneana is only 4 to 5” in diameter. 
* Die Schwimmpolypen v. Messina, 1858, p. 57, Pl. XII. 
