330 Dr. H. A. Nicholson on the Minute 



knowledge of the skeleton of excellently preserved recent and 

 fossil species of Heteropora, though we are still unfortunately 

 in total ignorance of the structure of the soft parts. 



My own object in writing the present paper is quite a 

 special one, and arises from the fact that the genus Hetero- 

 pora, apart from its own intrinsic interest, has a peculiar 

 importance in the eyes of palaeontologists, owing to the well- 

 recognized and remarkable external resemblance which it 

 exhibits to the Palaeozoic genus Monticulipora, d'Orb. So 

 striking is this resemblance that very high authorities have 

 employed it as one of their principal arguments for the 

 removal of Monticulipora and its allies bodily to the Polyzoa, 

 a transference which has been actually carried out in some 

 works of great weight (as, for example, in Prof. Zittel's 

 admirable ' Handbuch der Palaeontologie," vol. ii. Lief. iv.). 

 Not being myself, at present, prepared to acquiesce in the 

 removal of Monticulipora to the Polyzoa, and being in 

 possession of sufficient specimens of the recent Heteropora 

 neozelanica, Busk, I determined to investigate for myself 

 how far the resemblance between the two genera might 

 extend as regards the details of their internal structure. 

 With this view I prepared a series of thin sections of II. 

 neozelanica, and have carefully studied these and compared 

 them with precisely corresponding sections of various species 

 of Monticulipora. Tn the present paper, then, I propose to 

 give an account of the minute structure of the above-men- 

 tioned species of Heteropora and of two different types of 

 Monticulipora (selected for different reasons), comparing these 

 with one another, with the view of ascertaining how far they 

 may agree with, or differ from, one another in fundamental 

 characters. Before proceeding to this, however, it may be 

 advisable to make a few very brief and general remarks on 

 the genus Heteropora, and also to give a short account of the 

 external characters of H neozelanica, Busk. 



The genus Heteropora is thus defined by Prof. Busk in his 

 classical ' Monograph on the Fossil Polyzoa of the Crag ' 

 (1859) :— 



" Polyzoarium erect, cylindrical, undivided, or branched ; 

 surface even, furnished with openings of two kinds ; the 

 larger representing the orifices of the cells, ana the smaller 

 the ostioles of the interstitial canals or tubes." 



The essential character of the genus is thus the possession 

 of a skeleton made up of two kinds of tubes, larger and smaller, 

 the latter being the most numerous. The former have always 

 been regarded as the proper zooecia; but the relations of the 

 interstitial tubes or " cancelW' 1 to the rest of the organism 



