222 



ing from its inner margin, but a calcareous expansion is also formed under the 

 sternal area, which joins the calcareous lamina that closes the aperture. 



Of the numerous species of this genus we shall here only describe a few, 

 making one of them, Sciiticella plagiostoma, the object of a more detailed de- 

 scription. 



Scuticella plagiostoma Busk. 



Catenicella plagiostoma Busk, Voyage of Rattlesnake, pag. 358, Catalogue 



of Marine Polyzoa, Cheilostomata, pag. 8, PI. V, figs. 1, 2. 



(PI. XI, figs. 1 a-1 p). 



The asymmetrical, angularly oval zooecia have an oblique aperture, the length 

 of which is about one-third of the whole length of the zocecium and its anter 

 is almost semi-elliptical. From the two indistinct hinge-teeth the lateral margins 

 bend outwards, converging again proximally, and the aperture is by this means 

 provided with a short proximal expansion, which has a slightly convex margin 

 forming an obtuse angle with the one and an acute angle with the other lateral 

 margin of the aperture. The slightly chitinized operculum, surrounded by a 

 more chitinized marginal portion, does not fill the lateral parts of the proximal 

 expansion of the aperture and somewhat decreases in breadth from the hinge-line 

 towards the proximal end. 



The sternal area, occupying the greater part of the frontal surface, has 5 

 large, generally pear-shaped fenestras covered by a membrane. They are separated 

 by narrow ribs, which meet in the centre of the zocecium in an oblong sternal 

 portion. This often shows more or less distinct sutural lines as sign of the fusion 

 and such a faint sutural line, ending in a fine pore, is seen almost vertically on 

 the proximal margin of the aperture and a. little closer to the acute-angled corner. 

 It is due to the fusion of the two extremely small ribs, that fill the extremely 

 small sternal sinus. Inside the inner half of the fenestrae we find the marginal 

 •portion of the oblique, semi-elliptical, cryptocyst lamina, which springs from the 

 proximal margin of the aperture and originally hangs freely down on the inner 

 surface of the sternal area (fig. 1 n, c. pi.). In time however it coalesces partly 

 with the sternal ribs and partly with that cryptocyst, which gradually closes the 

 outer part of the fenestrse. The distal wall (fig. 1 n, dw.) is composed of a basal, 

 horizontal and a frontal, obliquely ascending part, of which the former is pro- 

 vided with numerous (up to 60) small, uniporous rosette-plates, disposed in a 

 horseshoe-shaped group with the opening towards the frontal side of the zocecium. 

 The ascending part is distinctly visible through the operculum at the distal end 

 of the aperture. 



