76 MR. A. W. WATERS ON 
PORELLA. 
The genus Porella is very well represented in the Arctic, 
and therefore some comparisons have been made with Porella 
from various localities, and the opercula and mandibles of some 
species not occurring in the present collection, as P. cervicornis, 
M.-Edw., P. levis, Flem., P. rostrata, Hincks, P. lorea, Alder, 
have been figured. As the result of these examinations, two 
eroups have been made, though itis felt probable that the second 
may be ultimately separated from Porella. 
The typical Porella may be adnate or erect, and has below or 
within the aperture an avicularium with a semicircular mandible, 
and this mandible has well-marked thickenings forming diagonal 
bars as mentioned by Busk *. 
The operculum is nearly straight on the proximal edge, with 
the corners cut off, and a muscular ridge on each side a little 
distance from the border. The ovicells are always or usually 
imperforate. In the group there is not usually any lyrula 
{denticle), though in P. concinna, Busk, and P. minuta, Norm., 
it can be found. Perhaps with more material the synonyms 
will be more reduced than I have been able to do in this paper. 
The second group has a very wide avicularian chamber, in 
which there is a pair of large glands. Where there has been 
suitable material these glands have been found. The operculum 
has muscular ridges farther from the edge than in the first 
group, and the mandibles have no marked cross-bars, but a 
lucida in the middle. ‘The mandibles are usually semicircular, 
but in P. acutirostris they are triangular. 
In this group are P. saccata, Busk, P. perpusillat, Busk 
(elegantula, d’Orb.); inflata, sp. nov.; P. acutirostris, Smitt, 
and apparently Perella marsupium, MacG.; perhaps Lepralia 
foliacea, B. & S., and P. margaritifera, Quoy & Gaim. 
In P. rostrata, Hincks, and P. malleolus, Hincks, the 
mandible spreads out at the distal end, aud this seems to be the 
case in some other Australian species, and occurs also in 
P. nitidissima, H.; but as P. rostrata has a distinct denticle, 
perhaps it should be put under Smittia. We may here remark 
that although the well-marked diagonal bars to the mandible are a 
noticeable character, yet the structure is not confined to Poredla, 
* Zool. Chall. Exp. pt. xxx. p. 147. 
+ P. perpusilla, Busk, from Newfoundland, has a denticle. 
