HYDROIDA. BALE. 289 



Sertularella tridentata, Billard, Ann. Sci. Nat., Zool., (9), 

 ix., 1909, p. 312 ; Id., Billard, Ann. Sci. Nat., Zool., 

 (9), xi., 1910, p. 14. 



Sertularella tridentata, Stechow, Abh. math.-phys. Klasse 

 K. Bayer Akad. Wissensch., hi. Suppl.-Bd., 1913, p. 

 137, figs. 111-113; (A 7 o. 17 Thuiaria sp., Inaba, Zool. 

 Mag. Tokyo, 1890, figs. 46-48). 



Of the foregoing, Sertularella tridentata, Lamouroux, is said by 

 Billard, who has examined the type specimens, to be identical 

 with the form described by him in 1907 as S. lata, hence he 

 has since considered the latter a synonym of S. tridentata. 

 Lamouroux' specimens, not including the gonangia, may be 

 doubtful, but if really identical with Billard's species, they 

 are not the true S. lata. 



Nutting says that the specimens assigned by him to S. lata 

 are the same as Allman's Thuiaria hyalina, types of which he 

 has examined ; the absence, however, in both cases of the 

 gonangia renders the identification doubtful. It should be 

 noted that Allman describes the hydranth of T. hyalina as 

 only partially retractile, the upper portion, including the 

 tentacular crown, being incapable of withdrawal into the 

 hydrotheca, even when most fully retracted. In S. lata, on 

 the contrary, the hydranths are found fully withdrawn into 

 the hydrotheca, and occupying little more than the lower 

 half of its cavity. 



Inaba's species, which Stechow has referred to S. tridentata, 

 is also without the gonangia. The figures resemble 8. lata, 

 but the aperture of the hydrotheca does not approximate 

 so closely to the vertical position. It cannot be identified 

 with 8. lata with any certainty. 



There remains Billard's 8. lata or S. tridentata, with 

 T. diaphana, Busk, S. Torreyi, Nutting, and S. speciosa, 

 Congdon, and it is probable that these forms constitute a 

 single species, the gonangia, as well as the hydrothecse, being 

 of the same type throughout. Billard's figure of the tropho- 

 some differs from S. lata in the hydrothecse being more bent 

 outward and having the apertures looking more upward. 

 The most distinctive feature of this form, however, as of the 

 others here associated with it, is the character of the gonangia. 

 These are cylindrical in the upper half and gradually tapering 

 below, not contracted at the summit, but even, according to 

 Allman and Congdon, somewhat flaring. Surrounding the 

 upper half are a number of longitudinal plications, between 

 which the wall of the gonangium is convexly curved outwards. 

 There is no trace of bilateral symmetry, and the form bears 



