Limitation of Genera among the Hydroida. 379 



the Medusa is produced directly from the egg without the interven- 

 tion of a polypoid tropliosome*. 



The absence of a trophosome affords a differential character of 

 much importance ; and the name of Lizzia must accordingly be con- 

 fined to such Medusae as, with the form of Lizzia, are directly deve- 

 loped from the egg, while it may be provisionally used for such as 

 resemble them, but have not yet had the other terms of their life- 

 series discovered ; the detection of these will determine the genus, 

 Lizzia or Leptoscyphus, or possibly some other still, to which the 

 provisionally named Hydroid may belong. 



Leptoscyphus tenuis, AUm. 



2. Lafoea, Lamouroux. 



Trophosome. — Hydrocaulus creeping and adherent, or erect 

 and rooted by a filiform hydrorhiza ; hydrothecse sessile on the 

 hydrocaulus, or pedunculated, tubular, destitute of an opercu- 

 lum. Polypites cylindrical, with the tentacular verticil surround- 

 ing the base of a spherico-conical metastome. 



Gonosome. — Gonophores phanerocodonic. Medusse deep bell- 

 shaped, with short manubrium, four radiating canals, and four 

 marginal tentacles with bulbous bases, destitute of ocelli ; tenta- 

 cles alternately long and short ; lithocysts not present. 



I have described the Medusa on the authority of Agassiz. It was 

 observed by Mr. A. Agassiz on a Hydroid which his father refers to 

 the Lafo'ea cornuta of Lamouroux. In this observation we have thus 

 a second example of the production among the Campanularida of 

 Medusae belonging to the type which is destitute of lithocysts and 

 developes its generative elements in the walls of the manubrium. 

 The first recorded instance was described by myself some years ago 

 in Leptoscyphus tenuis. 



Lafo'ea dumosa, Linn, (sp.), = Campanularia dumosa, Fleming, 

 Johnst., = Calycella dumosa, Hincks. 



Lafo'ea cornuta, Lamx. 



Lafo'ea fruticosa, Sars, = Campanularia ffracillima. Alder. 



Lafo'ea plicatilis, Sars. 



LineolaridaB. 

 1. LiNEOLARiA, Hincks. 



Trophosome. — Hydrocaulus a creeping and adherent tube 

 carrying the hydrothecse from distance to distance along its 

 length ; hydrothecse sessile on the hydrocaulus, tubular, with 

 the orifice subtriangular and armed with an earlike projection 

 on each side. Polypites not known. 



Gonosome. — Gonangia large, oviform, and adherent, sessile on 

 the creeping hydrocaulus. Gonophores not known. 



The genus Lineolaria was constituted by Hincks for a remarkable 

 * See Claparede in Zeitschr. f. wissens. Zool. Band x. 1860. 



25* 



