HYDROIDA. 199 



scribed by Mr Carter, the skeleton is calcareous. The earliest 

 known fossil forms of Hydractinia appear in the Cretaceous rocks, 

 and a number of Tertiary forms are known. In the Hydractinia 

 circumvestiens of the Pliocene Tertiary (which is probably identical 

 with the H plioccena of Dr Allman), the skeleton is calcareous, 

 and forms thick crusts upon the shells of Gastropods (fig. 79, a). 

 The genera Thalaminia (Jurassic and Cretaceous) and Spharactinia 

 (Jurassic) have been founded by Steinmann for forms apparently 

 allied to Hydractinia, and it is not impossible that the Jurassic 

 genus Ellipsactinia, of the same author, should likewise be placed 

 here. 



In many respects related to Hydractinia is the singular Mesozoic 

 genus Parkeria, best known by the Parkeria sphcerica of the Green- 

 sand of Cambridge. The organism in Parkeria sphcerica (fig. 80) 

 is globular in form, and varies in diameter from a quarter of an 

 inch or less up to more than two inches, the surface being often 

 nodulated or covered with small rounded or elongated elevations. 

 The skeleton must have been free in 

 its adult condition, as a rule at any 

 rate ; but in many cases it grew round 

 some foreign body, such as a frag- 

 ment of shell. As to its chemical 

 composition, there is no sufficient 

 reason to doubt that the skeleton VfJ 

 was originally composed of carbon- 

 ate of lime, though many specimens 

 are now found to be more or less 

 largely phosphatised. As regards * v -* 



its minute structure, the skeleton is „. . . , „ , . 



/ . . Fig. 80. — A large specimen ot Parkeria 



found, When examined miCrOSCOpi- sphcerica, from the Upper Greensand of 



ii 1 c ,, • .- . Cambridge, of the natural size. (Original.) 



cally by means of thin sections, to 



be composed of a characteristic minutely tubulated tissue, the 

 tubules of which are radial in arrangement. This tubulated tissue 

 is built up into a set of radiating columns ("radial pillars," fig. 

 81, /) and a series of concentrically disposed lamellae, the latter 

 being separated by interspaces (" interlaminar spaces "), which are 

 broken up into irregular " chamberlets " (fig. 81, c). The tubu- 

 lated tissue of the " radial pillars " is traversed by a variable number 

 of comparatively wide, circular or oval tubes, which open upon the 

 surface, or into the chamberlets — each successive row of chamber- 

 lets having at one time formed the surface of the skeleton — and 

 which may be regarded as having lodged the nutritive polypites of 

 the colony. Upon the whole, Parkeria may be regarded as repre- 

 senting an aberrant type of the Hydractiniidai, but the genus has 

 also relations to the Hydrocorallines. The little spheroidal fossils 



