12 SUEFACE 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 differing 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 fixr no attempt lias been made to comjjare them. I mean the 

 Hydrocorallinae. My basis for this comparison rests upon the presence of 

 the singular white plate of Kcilliker, and of its peculiar structure, — which 

 reminds us of the porous structure of the corallum of Sporadopora, Allopora, 

 Millepora, etc., 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 ftxct 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 Milleporid?e, which date back to the cretaceous period ; and, on the other, 

 the similar structure of the Helioporidje, which, as is well known, have been 

 shown by Moseley to he Halcyonoids. Whether the Stromatopora3 have any 

 relationship to either of these groups or are sponges cannot at pi^esent be 

 determined ; but should they be related to the Milleporidae, the peculiar 

 structure of the corallum of the Milleporidae, StylasteridiB, and Porpitidte 

 would date back to the earliest Silurian. It is interesting to speculate, if 

 the affinity of Porpita is greater to the Milleporidse with a porous fixed 

 corallum, or to the Tubularians having only a chitinous fixed basis. 



Porpita Linnaeana Less. 



Kolliker * 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 ly in diameter, while 

 the medium size of the P. Mediterraneana is only 4 to 5'" in diameter. 



• Die Schwirampolypen v. Messina, 1853, p. 57, PL XII. 



