﻿Probable Origin of the Hydroid Genus, Selaginopsis. 221 



XXI. Note on the Probable Origin of the Hydroid Genus 

 Selaginopsis. By James Eitchie, M.A., B.Sc, Natural 

 History Department, the Eoyal Scottish Museum. 



(Read 21st December 1908; received 22nd February 1909.) 



That the connection between Selaginopsis and Thuiaria is 

 a close one is evident from the similarity of their species in 

 general habit, from the resemblance of their gonangia in 

 shape, and from the almost universal occurrence in each of 

 a simple abcauline operculum. The degree of the relation- 

 ship, however, has been a matter of doubt. 



Prof. C. C. Nutting considers that "the affinities [of 

 Selaginopsis] are evidently with Thuiaria, from which, how- 

 ever, it is sharply differentiated by the 

 arrangement of its hydrothecse." ^ In 1902 

 Mr H. B. Torrey described as belonging 

 to the Thuiaria group of the genus Sertu- 

 laria a species, Sertularia incongrua — char- 

 acterised by possessing two rows of hydro- 

 thecse on the proximal portions of the 

 branches, three on the distal — which he 

 regarded as showing a "transition from 

 the Thuiaria to the Selaginopsis type."^ 

 Nutting, however, regards this species as 

 identical with the Selaginopsis triserialis of 

 Mereschkowsky, and seems to cast doubt on 

 the idea of intergradation, on account of the 

 fact that " several species of Selaginopsis 

 . . . have but two rows of hydrothecse on 

 the proximal part of their branches, the Fig. i. Abnormal 

 other rows being intercalated distally." 



Recently a slide containing a fragment 

 of a Hydroid specimen has been given 

 me by the Rev. J. Waterston, and this 

 appears to aid in linking together the two 

 genera. The fragment is part of the stem of a young 



1 Nutting, C. C, "American Hydroids. Pt. II., The Sertularidse, " 

 Smithsonian Institution, Special Bulletin, 1904, p. 127. 



2 Torrey, H. B., 1902, "The Hydroida of the Pacific Coast of North 

 America," Univ. Calif ornia Puh, , Zool., vol. i. p. 69. 



branch of 

 Thuiaria thuja 

 (L.), with three 

 series of hydro- 

 thecse. x 40. a, 

 upper surface. 



i 



