10 Prof. H. A, Nicholson oti the Structure and 



Bearing the above in mind, it is easy to understand the 

 phenomena presented by sections of Parkeria vertical to the 

 surface (woodcut, p. 6, and PI. III. figs. 2 and 4). All such 

 sections exhibit the radial pillars, the concentric lamella, and 

 the concentric rows of chamberlets ; and there is no difficulty 

 in the recognition of the nature and mode of origin of these 

 structures, if the plane of the sections corresponds accurately 

 with the direction of the radial pillars. Sections tangential 

 to the surface are not quite so easy of interpretation, though 

 they present no special difficulties. Such sections differ 

 according as the plane of the section corresponds with that 

 ot one of the concentric lamellse or with a row of chamberlets. 

 As the lamellae are curved, an ordinary tangential section 

 (PI. III. figs. 1 and 3) passes partly through one of the 

 concentric lamellse and partly through a tier of chamberlets. 

 Hence such a section usually shows more or less clearly the 

 cut ends of the radial pillars (jc) connected by a reticulated 

 tissue representing the transversely divided tubuli of the con- 

 centric lamellae. Here and there we also see irregular spaces 

 (c), which represent the chamberlets opened from above. In 

 places where the section corresponds with a row of chamber- 

 lets, we see simply tlie cut ends of the radial pillars. In either 

 case, tangential sections clearly exhibit the transversely- 

 divided zooidal tubes {t). 



6. Relation to Recent Organisms. 



With regard to the systematic position of Parkeria^ there 

 can be little hesitation, in the light of our present knowledge, 

 in accepting Mr. Carter's reference of the genus to the 

 Hydrozoa. All the known facts as to the chemical constitu- 

 tion, mode of growth, and general structure of the coenosteum, 

 no less than the minute structure of the skeleton-fibre, point 

 unequivocally in this direction. With regard to the precise 

 place which Parkeria should occupy in the series of the 

 Hydrozoa, it may be regarded as intermediate between the 

 Bydrocorallines and the Hydractiniidae, but with nearer 

 relationships to the latter than to the former. In the minute 

 structure of the skeletal tissue Parkeria most closely resembles 

 the Hydrocorallines ; but in the general arrangement of its 

 parts, and more particularly in its mode of growth by the 

 production of successive concentric lamellee separated by rows 

 of chamberlets, it approaches most nearly to the Hydrac- 

 tiniidaj, with which group the genus may in the meanwhile 

 be ranked. 



