SYSTEMATIC POSITION AND AFFINITIES. 65 



therefore, merely deal briefly with the evidence bearing upon their reference to the 

 Hydrozoa, and upon the position in that class which ought to be assigned to them. 



The first observer who seems to have suspected the relationship between the 

 Stromatoporoids and the Hydrozoa was Dr. Lindstrom, who pointed out that 

 Labechia, E. and H., previously regarded as a " Tabulate " Coral, possessed a 

 skeleton in many respects very similar to that of Ilydr actinia, Van Beneden (' Ofvers. 

 af Kongl. Vetenskaps-Akad. Fork.,' No. 4, 1873, and ' Ann. and Mag. Nat. Hist.,' 

 July, 1S76). The next observer who took up this subject was Mr. Carter, who 

 published a series of most valuable papers on the structure of the skeleton of the 

 Hydradiniidce (' Ann. and Mag. Nat. Hist.,' 1877 and 1878), and who maintained 

 that the Stromatoporoids were Hydrozoa and related to Hydradinia and also to 

 Millepora. In 1878 also, Dr. Steinmann published his admirable memoir ' Ueber 

 fossile Hydrozoen' (' Palaeontographica,' n. F., v. 3 (xxv), p. 101), in which he 

 not only referred Stromatopora itself to the Hydrozoa, but greatly increased our 

 knowledge of various related types. The views advocated by the observers just 

 mentioned have been since adopted by Zittel (' Handbuch der Palaeontologie '), 

 Roemer (' Lethaaa Palaaozoica '), Bargatzky (' Die Stromatoporen des rheinischen 

 Devons '), and other competent authorities, and may be regarded as now almost 

 universally accepted. This general acceptance of the reference of the Stromato- 

 poroids to the Hydrozoa is, perhaps, the more remarkable, when it is considered, 

 as before pointed out, that no demonstration had been effected of proper zooidal 

 tubes in any of the normal Stromatoporoids. At the present time, therefore, when 

 such tubes can be shown to exist in many forms, there can be little hesitation in 

 admitting the Stromatoporoids to a place in the class of the Hydrozoa, though 

 there may be some difference of opinion as to the precise position in this class which 

 they ought to occupy. 



In order to determine this last point, if only approximately, it will be necessary 

 to consider more particularly the structure of the skeleton in the two recent 

 genera of Hydrozoa which are most nearly related to the Stromatoporoids, viz. 

 Hydradinia and Millepora. 



Hydradinia echinata, Flem., the most readily obtainable type of the genus 

 Hydradi?iia,iorms thin horny crusts, which grow upon the exterior of various Gaster- 

 opodous shells, but apparently only upon those which are tenanted by Hermit Crabs. 

 In its earliest condition, the skeleton consists of a delicate chitinous pellicle, growing 

 upon some shell, by the maceration of which in weak acid it can be readily obtained 

 for examination. In this stage it consists of numerous nodal points, the so-called 

 " horn-cells " of Carter, united by radiating horizontal processes, or fibres, which 

 coalesce to form an irregular cribriform membrane, for which we may employ Mr. 

 Carter's name of the " basal lamina " (Plate VI, fig. 2). According to Mr. Carter's 

 researches, the " horn-cells " appear first in the substance of the shell as separate 



