378 ACAULIS PRIMARIUS. 



In the above diagnosis I have confined myself to one of the forms described by Stirapson, as 

 two different states of the same hydroid, and I have done so because I feel sure that Stimpson 

 had two entirely different hydroids under examination when he believed that he had only two 

 different phases of a single one. 



After describing an animal with the characters given above, and which he found floating free 

 in the open sea, Stimpson goes on to say : — " At a subsequent time I met with several of these 

 animals, which presented a diff'erent appearance. The tentaculae were larger, especially in the 

 region of the mouth, at the now blunt extremity of the body, and the medusa-buds were in an 

 advanced state of development, soon to become free-swimming individuals. The inferior appen- 

 dages had disappeared, and the body was firmly attached by a broad base, and bore much resem- 

 blance to one of the ordinary Cori/itulce deprived of its stalk. In strong contractions it assumed a 

 shape resembling that of an hour-glass. The length of the animal in this latter stage was half an 

 inch, the breadth two tenths. In the earlier stage the dimensions were one half these. 



" It was dredged in the Laminarian zone, from five to fifteen fathoms, attached to various 

 Rhodosperms, as Ptilota, Chondrus, and Hhodymena" 



The animal thus described is a Suncorifne-XAsS, hydroid with undeveloped stem, and has 

 assuredly nothing to do with the Tuhidaria-\'k% free hydranth previously described. If I am 

 correct in this supposition, the Syncorjjne-\k.Q fixed hydroid presents us with a distinct generic 

 form, characterised by its sessile hydranths with scattered capitate tentacula and phanerocodonic 

 gonophores. It is not improbably identical with the genus IlaJocharis of Agassiz ('Contr. Nat. 

 Hist./ N. S., vol. iv, p. 239, pi. xx, fig. 10), a genus from which he afterwards withdrew the 

 name of Halocharis in favour of M'Crady's name of Cori/nites. (See the description of Conjnifes 

 given above.) 



Stimpson found the animal for which he constituted his genus Acaidis in the condition of a 

 free floating hydranth, and derived fi'om this fact the leading character of the genus. I am 

 strongly of opinion, however, that, with the similar condition attributed by M'Crady to 

 Nemopsis, this free state is only accidental, and that the floating hydranth, with its gonophores, 

 had been detached from a stalk in a way we know to be so common in Tulularia. I have 

 therefore not included the freedom of the trophosome as a character in the diagnosis. 



*^* AcAULis puiMAEius, Stimpson. 



AcAULis PRi.MAKius, — Stimpson, Marine luvertebrata of Grand INIanan, pi. i, fig. 4, in Smith- 

 sonian Contributions, vol. vi, 1854, Agassi::, Contr. Nat. 

 Hist. U.S., vol. iv, p. 315. Allman, Ann. Nat. Hist, for 

 May, 1861. 



TROPHOSOME. — Hydranth, with the tentacles of its proximal zone, eight in 

 number ; distal tentacles numerous, very small, commencing a little below the mouth, 

 and thence scattered over about two tliirds of tlie surface of the body. 



