﻿222 Proceedings of the Royal Physical Society. 



pinnate colony of Thuiaria thuja (linn.) ^ bearing fifteen 

 alternate branches, twelve of which are normal in every 

 respect. The remaining three follow each other on one 

 side of the stem at its distal end. Of these the proximal 

 two are typical in structure, but abnormal in orienta- 

 tion, for the hydrotheca rows, instead of lying in the 

 vertical plane which contains stem and branches, lie 

 in a horizontal plane cutting the stem at right angles. 

 The rows, as if to prepare for the next step in a pro- 

 gressive variation, are placed rather on the lower side of 

 the middle of the branch. The abnormal branch last to 

 develop resembles its immediate predecessors except that an 

 additional row of hydrothecse has been interpolated on the 

 upper surface. This branch, then, exactly resembles those 

 of Selaginopsis triserialis where three series of hydrothecee 

 are present. Further, Bale has described and figured a 

 similar variety of the Australasian Thuiaria (^Sertularia) 

 unguiculata (Busk), a species which belongs to the genus 

 Thuiaria — as defined by Nutting. 1904 — on account of the 

 presence of more than one pair of hydrothecse on many 

 internodes, the hydrothecae, where several occur together, 

 assuming an alternate or sub-alternate arrangement. The 

 most remarkable feature of this variety "is the presence 

 on some of the pinnae of a third series of hydrothecae, 

 running for some distance along the front of the first 

 internode," and Bale regards this as showing a transition 

 towards Selaginopsisr While great stress cannot be laid on 

 an occasional meristic abnormality, these variations, taken 

 together with the existence of such a form as S. triseHalis, 

 seem to point to one of the paths which phylogenetically 

 connect ThuiaHa with Selaginopsis. Since, moreover, 

 Selaginopsis colonies pass through a Thuiaria-stage in early 

 development (they, indeed, always retain a Thuiaria stem), 

 and since they are more liable to variation — less stable in 

 structure — than Thuiaria colonies, it may be assumed that 

 of the two types Thuiaria is the older and better established, 

 and that therefore from it Selaginopsis has been evolved, 



1 Dredged by Mr Waterston, off the Bell Eock, SOth August 1904. 



2 Bale, W. M., Catalogue of the Australian Hydroid Zoophytes. Sydney, 

 1884, p. 77, pi. \d. fig. 9. 



