1 84 Appendix to Part L 



about an inch, in breadth and half an inch in height, and consists of 

 two layers of net-work, — one presenting quadrangular meshes, and 

 the other hexagonal, with a round, inner area ; and over a consider- 

 able part of the specimen, the quadrangular net-work has been 

 removed, whereby the connexion of the two structures is perfectly 

 exposed. 



This fossil is believed to agree completely in its essential generic 

 characters with those of Hemitrypa (' Pal. Foss. Cornwall,' p. 27), 

 but its state of preservation, and some facilities afforded by it for 

 determining structural details, have led to an inference respecting 

 its nature somewhat different from that given in the work just 

 quoted. 



The inner surface of Hemitrypa oculata (loc. cit.) is described as 

 ' marked with radiating ridges," having intervening ' oval depres- 

 sions, which penetrate only half through the substance of the coral, 

 and nowhere reach the outer surface.' The equivalent portion of 

 the Van Diemen's Land specimen agrees perfectly with this state- 

 ment, except in the form of the meshes or depressions ; it is, how- 

 ever, not merely 'like some Fenestellae,' but it possesses all the 

 essential characters of that genus, and is believed to be a fragment 

 of Fen. fossula. This inference is drawn from a minute portion 

 mechanically detached, and which exhibited a row of large, round, 

 projecting, cellular mouths. The external surface of Hem. oculata 

 is described as ' wholly covered with numerous round pores or 

 cells ' — ' associated in double rows,' and the corresponding portion 

 of Hem. sexangula has been ascertained to consist also of a similar 

 surface of double rows of round meshes or ' pores,' but with hexa- 

 gonal boundaries; and they are shown, as exhibited by the speci- 

 men in its imbedded state, to penetrate to the surface of the Fene- 

 stella or quadrangular net-work. 



These details are conceived to be sufficient to establish a generic 

 agreement between the Van Diemen's Land coral and Hemitrypa 

 oculata ; and an examination of an Irish specimen of that genus 

 has fully confirmed the structural details exhibited in the 'inner 

 surface ' of the specimen to which, provisionally, the name of Hemi- 

 trypa sexangula is applied. 



Of the true nature of the ' external ' net-work no opinion is 

 ventured. It is formed almost entirely of dark gray, calcareous 

 matter, tilling apparently an originally cellular structure ; but there 

 are also a few small patches of the outer covering, consisting of an 

 opaque white crust on the surface, which was originally in contact 

 with the external net-work. That it was a parasite little doubt 

 is entertained ; and the interesting agreement between the space 

 occupied by the double row of meshes, and that of the parallel 

 branches of the Fenestella, arises apparently from the latter having 

 afforded suitable base lines for attachment. In the Van Diemen's 

 Land specimen, the agreement is marked by an increased breadth in 

 the net-work, and by a row of projecting points. There is also a re- 

 markable asreement between the arrangement of the mouths of the 



