526 Eev. T. Hincks on British Polyzoa. 



in certain general characters, but many of them distinguished 

 from tlie rest by differences of very great significance. For 

 example, L. w^V^c?a, Johnston, and L. Peachiiy Johnston, agree 

 in being adnate and incrusting, and in having cells which are 

 calcareous, decumbent, and contiguous ; but they differ widely 

 in the structural composition of the cell (the plan upon which 

 it is built) ; and to group them together is simply misleading. 

 The definition of the genus, in short, is too general and vague. 

 To make any approach to a natural system our groups must 

 be founded on a careful and minute study of the individual 

 cell in its various stages of growth. 



The following seem to me to constitute natural and well- 

 defined groups : — 



1. Membraniporella, Smitt. 



{Lepralia, part, Johnst., Busk, &c.) 



Zoarium incrusting ; zoooecia closed in front by a number 

 of flattened calcareous ribs, more or less consolidated. 

 Type Lepralia nitida^ Johnston. 



2. Cribrilina, Gray. 

 (^Lepralia, part, Johnston &c.) 



Zoarium incrusting ; zoooecia in a single layer, contiguous, 

 having the front more or less occupied by transverse or 

 radiating punctured furrows. 



Type Lepralia radiata^ Moll. 



3. MuCRONELLA, nov. gen. 

 {Lep-alia, part, Johnston &c.) 



Zoarium incrusting ; zoooecia usually ovate, punctured round 

 the base, with a suborbicular or semicircular aperture, the 

 inferior margin mucronate, a denticle within it ; avicularia 

 generally wanting. 



Type Lepralia Peachiij Johnston. 



4. MiCROPORELLA, nov. gen. 



{Lepralia, part, Johnston &c. 

 Porellina, Smitt. 

 Reptoporina, part, D'Orbigny.) 



Zoarium incrusting ; zoooecia with a semicircular aperture, 

 the lower margin straight and entire ; a semilunate or circular 

 pore below it. 



Type Lepralia ciliata^ Pallas. 



The Reptoporina of D'Orbigny includes this group j but aa 



