76 AMERICAN HYDROIDS. 



Hincks (istiS) described six, and Ilartlaub (I'.Hto) includes eighty -six species, the largest number 

 yet included in any one genus of the Hydroida. Allman adopted the genus in his earlier works, 1 

 but abandoned it in his Report on the Hyclroida of the Challenger Expedition, 1S88. Hincks, in 

 his British Hydroid Zoophytes, 1808, and Bale, in his Catalogue of Australian Ilydroid /oophytes, 

 1S84, called attention to the character of the opereulum, now considered the best means of delim- 

 iting the genus, and in this they have been followed by nearly all subsequent writers of 

 importance. 1 



In 1900 there appeared a work by Doctor Ilartlaub which contains by far the most complete 

 and masterly discussion that has ever been offered concerning any single genus of the order 

 Hvdroida.' 1 The ground has thus been so completely covered by one fully equipped for the best 

 work, with unusual advantages for examining a, large number of types in the most important 

 museums of Europe, that the present writer has found his labors greatly lightened so far as the 

 genus Xfi'tii/in-i'llii is concerned, and has availed himself freely and with confidence of the results 

 of Doctor Hartlaub's labors, especially in the matter of bibliography and distribution of species of 

 this great genus. In the few cases where Doctor Hartlaub has given references unavailable to 

 myself 1 have taken them on his authority, having verified so great a majority that I aiu con- 

 vinced of their entire reliability. This writer's definition off the genus is substantially (lie same 

 as the one adopted in the present work. He does not claim to give a final definition, but simply 

 employs it as a means of assembling all the forms that he regards as closely related to S^rfnlnri-Jln 

 as ordinarily understood. He finds that the characters of the internodes relied upon by Schneider* 

 is verv inconstant, and that Levinsen has relied too implicitly on the characters of the marginal 

 teeth and opereulum. 5 He does not regard the opereulum of Xi'/inlar<l/<t as homologous with 

 that of Si-rfiiliirin* the former being a definite structure added above the real hydrothecal margin, 

 while the latter is the thin end of the hydrothecal wall itself. 



POINTS OF INTERGRADATION BETWEEN SERTULARELLA AND OTHER GENERA. 



Although there are certain species that do not come strictly within the definition above given 

 for Seriularetta, there is only one that comes within the, limits of any other genus as defined in 

 this work, and that is .S'. i-lurk!!, which bears considerable resemblance to certain species of 

 TJniiitrld' in the aperture and marginal teeth. The strictly alternate liydrofhecie, however, and 

 the regularity of the internodes are sufficient, in my opinion, to overbalance these thuiarian 

 features and justify the, inclusion of the species in ,SV iinl<ir<lln. 



There are several species in which the gonosome differs from the typical S< /'tnlnrcJIn in not 

 being annulated. X. , /i/xmjii/x has gonangia that are strongly ribbed longitudinally, reminding 

 one of certain species of Abii'tiiitii-ii.i, such as A. cantata Nutting; S. f/t/'<//</// (Trask) has gonangia 

 that have, the distal portion beset with strong spines, reminding one of these structures in some 

 members of the genus Dt/phasia, such as ]>. acanthocarpus (Busk), and S. furnn<i.i Fewkes has a 

 perfectly smooth gonangium resembling some of those found in the genus X, I'tnlnr'xi. All of 

 these species, however, come well within the genus and could not consistently be placed in any 

 other. 



The, following kev, like the others in this work, is purely artificial and makes no claim to 

 indicate, the relationship of the species involved. 



'Diagnoses of New Genera anil Species of Hydroids, Journal of the Linn;i>an Society, 1S74, \\. 261 ; Memoirs of the 

 Museum of Comparative Zoology, V. No. 2, 1.S77, p. 21; Description of Australian, Cape, and other Hydroids, mostly 

 from the collection of Miss II. Hatty, Journal of the Linnrcan Society, XIX, 1885, p. 133. 



2 See the General Systematic Discussion, p. lit. 



3 Revision der Sertularella-Arten, von Doctor Clemens Hartlauli, Hamburg, 1900. 



4 Ilvdroidpolypen von Rovigno, Kiel, 1897, p. 523. 



5 Meduser, Ctenophorer og Hydroider f ra Gronlands Vestkyst, 1893, p. 57. 



