REPORT ON THE POLYZOA. 
107 
15. Retepora plana, Hincks. 
16. ,, tessellata, Hincks. 
17. ,, robusta, Hincks, = (?) Retepora porcellana, Macgillivray. 
To which may be adcled- 
18. 
19. 
20. 
21 . 
22 . 
23. 
24. 
25. 
26. 
27. 
28. 
29. 
30. 
31. 
bi-avicularia, Smitt, = Retepora beaniana, var. 
altisulcata, Ridley. 
microthyris, Busk (MS.). 
umbonata, Macgillivray (var. of monilifera). 
sinuata, Macgillivray ,, ,, 
lunata, Macgillivray (?) ,, ,, 
acutirostris, Macgillivray (?) (var. of monilifera ). 
munita, Hincks (?) ,, „ 
formosa, Macgillivray. 
carinata , Macgillivray. 
serrata, Macgillivray. 
aurantiaca, Macgillivray. 
laxa, Hincks (? var. porcellana, Macgillivray). 
avicularis, Macgillivray. 
To this list may be added, from the Challenger Collection, nearly as many more forms, 
thus raising the known, or approximately known, recent species of Retepora to between 
fifty and sixty. In view of this large number, it becomes almost imperative to subdivide 
the so-termed genus into sections, each having certain characters in common, and which in 
a monograph of the genus might perhaps come to be regarded as sub-genera, if such a 
term has any definite meaning. 
AYith respect to the most convenient mode of arranging the various species, I quite 
agree with Mr. Hincks in regarding it “ as more than doubtful whether the reticulated 
character of the zoarium is alone sufficient to supply the basis for a generic group.” This 
is in fact abundantly clear from the circumstance that there are not only among the 
Cheilostomata fenestrate forms, such as Adeona, and one or two other Escharidans, but 
also among the Cyclostomata, as in Retihornera. I have therefore not hesitated to include 
in the same generic group species in which there is either no reticulation of any kind, 
or one of a different character from that which obtains in the great majority of species. 
But at the same time a transition, as it may be termed, can be observed from the freely 
ramified forms to those offering a true reticulation. For, in a certain number of species, 
the branches or trabeculae, though lying mainly in one plane, are either quite free 
or irregularly interlaced, or united either by occasional direct anastomoses, or sometimes 
by non-celliferous transverse trabeculae. In the latter case the general disposition of the 
trabeculae or branches may be roughly likened to the tracery of Gothic windows of 
