341 



the peculiar large avicularia, which Hincks took to be occcia. The broadest 

 sinus is found in S. collaris and .S. Pallasiana, in which two species it is the 

 broadest part of the aperture. The operculum, which is never provided with mus- 

 cular ridges, is in most species membranous or feebly chitinized and very often 

 not or only indistinctly separated from the compensation-sac (e. g. in S. Smitti, 

 S. Lansboroui, S. reticulata, S. trispinosa), while in other species such a separation 

 is brought about either by its stronger chitinization (e. g. in S. propinqiia et S. 

 Im-ealis) or by the development af a chitinous sclerite along its proximal margin 

 in continuation of the opercular arch (e. g. in S. solida, S. reticulato-punctata, 

 S. collaris, S- Otto-Muelleriana, S. foliacea and S. Pallasiana). 



The avicularia may occur in very different positions and we can sometimes 

 hnd both two lateral as well as a single median avicularium in the same species 

 (e. g. S. trispinosa and S. linearis). This occurs for example in the same colony 

 of the above-mentioned S. linearis from Syracuse. In this genus however there is 

 usually a sometimes symmetrically, sometimes asymmetrically placed, median 

 avicularium proximaily to the aperture, and it may be noted as a contrast to 

 the corresponding avicularium in the genus Porella, that the median avicularium 

 in Sniittina has its frontal area as a rule parallel to the surface of the zoa>cium. 



Whilst the ectoooecium is as a rule provided with numerous pores, the number 

 of these may fall to 3 or 2 in vS. trispinosa and the same number is also found 

 in S. foliacea. In .S. arctica there is only a single pore, and finally pores are quite 

 wanting in S. Smitti and S. majnscula. An ooecial cover is present in most species 

 but in very different development, sometimes only forming a marginal belt (e. g. 

 in S. Lansboroui, S. borealis, S. collaris), sometimes concealing the whole frontal 

 wall of the ooecium and developing together with it (e. g. in S. Smitti, S. arctica, 

 S. majnscula). Its appearance in .S. trispinosa var. cncuUana (PI. XIX, fig. 7 a) is 

 characteristic, as it is provided there with a freely projecting, prominent margin, 

 as also in N. foliacea (Fl. XXIV, fig. 5 a) where it consists of three parts, which 

 are separated by two sutural lines converging towards the aperture. The middle 

 part belongs to the distal zocecium, whilst the two lateral parts belong to the 

 two neighbouring zoa>cia, and the two characteristic, large, flatly triangular pro- 

 jections, which partially cover the aperture of the ooecium-bearing zocecia, are 

 directly connected with the lateral parts of the ooecial cover. Only the proximal 

 part of the ooecium is covered in S. solida. 



Though calcareous ooecia have not been found in >Lepralia^ Pallasiana I must 

 refer this species to the present genus on account of the likeness it shows to 

 S. Otto-Muelleriana in the structure of the aperture and the operculum, in the 



