Glassification of the British Polyzoa. 159 
prevails, as in Lepralia Landsborovii , Johnston, in which the 
relation of the three modes of growth may be well studied ; 
in others the foliaceous, as in Eschara foliacea auctt. These 
differences involve neither change in the plan of gemmation 
nor any other structural peculiarity, and are really quite 
immaterial. 
The following species belong to the genus Lepralia :—• 
L. Pallas iana, Moll. 
L. canthariformis , Busk. 
L. foliacea , Ellis and Solander. 
L. pertusa , Esper. 
L. adpressa , Busk. 
L. hippopus, Smitt. 
L. edax , Busk. 
?L. polita, Norman (a somewhat aberrant form). 
PORELLA, Gray. 
Zooecia with the primary orifice semicircular ; secondary 
(or adult) orifice elongate, inversely subtriangular or horse- 
shoe-shaped ; an avicularium, usually with a rounded mandi¬ 
ble, within the lower margin. 
I place in this genus the following :— 
P. concinna , Busk. 
P. minuta, Norman. 
P. struma , id. ( Hemeschara auctt.). 
With erect zoarium. 
P. compressa , Sowerby ( —Cell . cervicornis , Johnston). 
P. Ice vis, Fleming. 
All the species here associated possess zooecia which are 
essentially identical in the adult state, and pass through the 
very same course of development. A minute and careful 
study of all the forms has convinced me that, so far as the cell 
is concerned , they are most intimately connected, and that 
none but specific distinctions exist amongst them. The 
various phases of the cell-growth correspond throughout the 
series. Porella concinna and Porella comp>ressa ( cervicornis ) 
have precisely similar structural elements : their habit of 
growth is dissimilar. The question arises, Is the minute 
agreement of the cells, or the difference in their grouping the 
most important point? Is it more philosophical to unite them 
in one genus on the strength of their structural similarity or to 
separate them for their diverse habit? If they are separated, 
it must be on the single ground of the difference in the 
