150 Rev. T. Ilincks on the 



extremity of the maroinal curve, peristome not raised ; imme- 

 tliatoly below the orifice an elongate linguiform avicularium, 

 about a third of the cell in length, or sometimes (in the case 

 of the dwarfer cells) about half the length, mandible pointing 

 downwards. Ocecium broader than high, rounded above, 

 and narrowing slightly towards the front, surface smooth and 

 silvery, a number 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. 



Bange. JSpitzbergen ; Greenland; Finmark; St. Law- 

 rence; Barents Sea; Grand 3Iaiian. 



The smaller form to which I have referred as occurring iti 

 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 signiticance. 

 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 (PI. YIIL fig. 1 h). In the smaller form the orifice 

 is rather taller than broad, the u])per margin moderately 

 arched, and the sides slightly curved, while in the centre of 

 the interior margin there is a small but distinct rounded sinus 

 (PI. YIIL fig. 1 a). 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 

 clher, and generally of a very different character. Notwith- 

 standing these important differences it is difficult to believe 

 lliat 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 

 ilie avicularia on the front wall, which produces an effect on 

 the general aspect out of all proportion to its intrinsic import- 

 ance. In the smaller form they arc 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 EscharcUa pahnata, which was founded on 

 the larger of the two forms with which 1 am dealing. His 

 figures represent only a sub(]uadrangular orifice. The 



