6'J AMERICAN HYDROIDS. 



hvdrothe.e.;v have two tenth und a two-Mapped opeiruhun and part no tootli and a single-flapped 

 operculum. 



The ifeniis, as I have detined it, is confessedly the least natural and satisfactory of those 

 admitted in this work. 



RUNTS OF INTERGKADATION BETWEEN THUIAKIA AND OTIIEI: < JKN'KKA. 



First. With S^rliiliirin, in having the hydrothecal margin with two teeth and a two-flapped 

 operculuni, as in T. iti'i/i'iif-n, T. x/////7/.v, T. t< in i-<t, T. f<il/ri<-ii. and /'. < upressina. In all of these 

 cases there are many liydrothecte to an internode, and they are not strictly opposite. 



Second. With Al>ii't!inir!ii,\\\ having an adcauline operculum, as in T. t/ni/<iri<>!<lix Clark. In 

 every other respect, both in trophosouie and in goiiosoine, this species is a typhical Tkularla. 



KEY T" AMERICAN SI'ECIES, UK TIII'IAKIA. 



Ilydruthecal margins smooth, except on distal ends uf branches, where they are bideritate. Operculum of one 



abcauline Hap. 1 

 Branches springing from all sides of stem. 



Colony in the form of a stiff " bottle brush" tlniju. 



Colony not so stiff and rigid. 

 HydrothecM' alternate. 



Margin produced on outer side into prominent recurved hooks flnjuiin. 



Margin smooth, except on distal parts of branches, where there are two strong teeth rolmnln. 



Hydrothec;e sub-opposite, margin smooth, operculum adcauline thuiarwides. 



Branches alternate. 



Hydrothecae opposite. 



Aperture round, facing directly outward po/i/mr/m. 



Aperture facing outward and upward, margin produced into a prominent lobe on adcauline 



side and appressed to stem knnl:i . 



Hydrothecie alternate. 



Hydrotheca' entirely immersed, aperture strictly vertical iiiiincivn. 



Distal end of hydrothec:e free, aperture facing outward and upward loiichiti*. 



Hydrothecal margin toothed, operculum with two flaps. 

 Hydrothecie strictly alternate. 



Branches opposite plumulift rn. 



Branches not. opposite. 



Margin with two large pointed teeth. 



Entire adcauline livdrothecal wall adnate ramosissima. 



I listal half < if adcauline wall free diffmai. 



Hydrotheca' sub-o]i]iosite, marginal teetli two, opposite. 



No hydrothecif on stem i/n/li. 



Hydrotheca on stem. 



HydrothecM- flask-shaped, their long axes not parallel with stem. 



Hydrothecae with iinequal teeth liitimu'ii/n. 



H yd re it 1 1 ecu with eiiual teeth simi/ix. 



Hydrothec:e tubular, their long axes parallel with stem tiibul!furini.i. 



Hydrotliecie suh-ojiposite, one or two marginal teeth. 



Branches alternate, and usually undivided, hydrotheese not immersed tiiurn. 



Branches in a sjiind, usually diviiled dichotomously. 



Spiral close, colony forming a ilense tuft .J'n/n-ii'ii. 



Spiral loose, hydro! hec.-e not greatly immersed ari/i nli-n. 



Spiral loose, hydrotheca- extensively immersed, their axes not parallel with stem cupressina. 



Spiral loose, hydrothecre extensively immersed, their axes parallel with stem jjlumosa. 



THUIARIA THUJA (Linnaeus). 

 (Plate VII, rigs. 1-3.) 



l!niili--/,riixli <'nrii//iiii- KI.I.IS, ICssay Nat, Hist. Corallines, 17."i5, p. 10. 

 Si'rtnlni-iii l/ni/ii I, INN. ECS, Systema Natune, 175S, p. soil. 

 Sfrliil<iri<i l/nijit lloi-n-cvx, N:itimrl\ke Historic, 17IH-177:!, p. 543. 

 St'fliilnfiii Iliii/n I'AI.I.AS, Klenchns /o.iphytorum, 176li, p. HI). 

 s,','tii/,n-iii Iliii/n LINN.F.CS. Systema Natune, 12th ed., 1767, p. 1308. 



1 Except in the case of T. thuiarifiidi-s Clark. 



