150 Rev. T. Hincks on the 
extremity of the marginal curve, peristome not raised ; imme- 
diately below the orifice an elongate linguiform aviculartum, 
about a third of the cell in length, or sometimes (in the case 
of the dwarfer cells) about half the length, mandible pointing 
downwards. Oecium broader than high, rounded above, 
and narrowing slightly towards the front, surface smooth and 
silvery, a numbker of roundish perforations and (in some 
cases) of narrow elongate fissures round the lower part of it, 
the central portion entire, or with a few scattered punctures. 
Range. Spitzbergen; Greenland; Finmark; St. Law- 
rence; Barents Sea; Grand Manan. 
The smaller form to which I have referred as occurring in 
the St. Lawrence is characterized by a very slender habit 
and by the narrowness of the segments which compose the 
zoarium. ‘These differences might only indicate an earlier 
stage of growth, but there are others of more significance. 
There is a remarkable dissimilarity in the shape of the orifice. 
Tn the larger form (of which we have an admirable descrip- 
tion from the elder Sars) it is subquadrangular (‘‘ rotundato- 
quadrangularis,” Smitt), and the lower margin is occupied by 
a wide and shallow sinuation, stretching between the articular 
denticles (Pl. VIII. fig. 16). Inthe smaller form the orifice 
is rather taller than broad, the upper margin moderately 
arched, and the sides slightly curved, while in the centre of 
the inferior margin there is a small but distinct rounded sinus 
(Pl. VIII. fig. 1a). I was at first inclined to think that the 
latter might be a merely peristomial structure ; but on detach- 
ing the opercula they were found to be furnished below with 
a projecting process corresponding exactly with the sinus. 
The orifice in this form is much more slender than in the 
other, and generally of a very different character. Notwith- 
standing these important differences it is difficult to believe 
that we are dealing with distinct species when we remember 
the remarkable similarity between the two forms in most of 
their characters. There is a difference, it is true, between 
the avicularia on the front wall, which produces an effect on 
the general aspect out of all proportion to its intrinsic import- 
ance. Inthesmaller form they are of very moderate size, and 
either circular or oval; in the other they are (prevailingly) 
large and linguiform. But such varieties of shape are too 
common amongst avicularian structures to have much syste- 
matic weight. Smitt long ago noticed the variability of the 
avicularium in his Escharella palmata, which was founded on 
the larger of the two forms with which I am dealing. His 
figures represent only a subquadrangular orifice. The 
