156 CCELENTER ATA. 



with a calcareous polypary have been described by Mr 

 Carter. 



The most important genus of Corynida which has been 

 certainly detected in the fossil condition is Hydractinia, which 

 is still represented by living species. The recent Hydractinia\ 

 as a rule, are horny as regards the texture of the skeleton, 

 and form crusts attached to the outer surface of shells. By 

 age, these crusts come to be composed of successive close-set, 

 vertically superimposed laminae, and the shell upon which 

 they originally grew is commonly more or less dissolved 

 away and replaced by the substance of the parasite. In an 

 African Hydractinia, described by Mr Carter, the skeleton is 

 calcareous, but not essentially different to the horny forms in 

 minute structure. Several fossil forms of Hydractinia are 

 now known, two of them (H. cretacca and H. Vicaryi) being 

 from the Upper Cretaceous system. Of these ancient types, 

 the latter is described as being siliceous, but it is more prob- 

 able that its skeleton was originally calcareous, and that it 

 has been silicified. In the Miocene Tertiary another species 

 occurs ; and in the Pliocene (Coralline Crag of Suffolk) we 

 have another species, in which the skeleton is calcareous, 

 Mr Carter's discovery of a living calcareous Hydractinia 

 rendering it probable that the fossil form possessed a skeleton 

 primitively composed of carbonate of lime. The genera 

 Thalaminia (Jurassic and Cretaceous) and Sphceractinia 

 (Jurassic) have been founded by Steinrnann for forms sup- 

 posed to be allied to Hydractinia. 



According to Mr Carter, the fossils which have been usually described 

 under the name of Strcmatopora (see p. 137), together with the large 

 arenaceous Foraminifera described by Dr Carpenter under the title of 

 Parkeria, are really closely related to Hydractinia, and are truly fossil 

 Hydrozoa. So far as Stromatopora and its allies are concerned, the author 

 is unable at present to accept this view, which appears to be founded upon 

 resemblances of analogy rather than of real and fundamental likeness. 

 It is not impossible, however, that Lindstrom is correct in regarding 

 the Silurian genus Labechia which has generally been regarded as a coral 

 as a close ally of Hydractinia. In this ancient and singular type we 

 have flattened calcareous expansions (fig. 47), the upper surface of which 

 is studded by blunt spines. These spines are seen in vertical sections 

 (fig. 47, c), to be the summits of perpendicular pillars, the spaces be- 

 tween which are occupied by vesicular calcareous plates. The chief 



