GENUS PLATYLEPAS. 425 



one end of the orifice to the other. The rostrum is much 

 broader than the carina : the lateral compartments are but 

 little broader than the carino-lateral compartments. The 

 radii are narrow, though variable in width. The general 

 shape of the shell varies much in the same species, being 

 either much depressed, or rather steeply conical ; either cir- 

 cular, or more commonly oval. The (so called) epidermis is 

 usually persistent in the lower part of the shell. The shell 

 itself is white. The basal diameter of the largest specimen 

 of P. bissexlobata was nearly three-quarters of an inch. 



Structure of the Parietes, Radii, 8fc. — The walls of the 

 shell in P. bissexlobata are permeated by minute pores, and 

 have exactly the same structure as in Coronula ; the pores 

 being completed by the union of ledges on the outer sides 

 of the longitudinal septa, and the latter have little knobs 

 formed on them at each period of growth. In P. decorata 

 the walls are solid. Each compartment, as already stated, 

 has a deep nearly medial furrow, the sides of which are 

 closely pressed together, thus forming the midribs, and 

 thus differing from the furrows in Coronula, which not being 

 pressed together, form cavities on the under side of the 

 shell. The inner longitudinal margin of each midrib is 

 thickly coated by a layer of shell, and is thus rendered much 

 more prominent than it would have been if formed exclu- 

 sively by the inward folding of the wall. The upper part of 

 each midrib forms a slight, longitudinal ridge (fig. Id) on 

 the sheath, having become encased by the sheath during its 

 downward growth. Owing to the small size of the mid- 

 rib of the rostrum, there is scarcely any ridge on the sheath 

 of this compartment. The sheath does not descend half- 

 way down the walls in P. bissexlobata, but farther down 

 in P. decorata ; but perhaps the thickened inner margins 

 of the midribs may be considered as the downward pro- 

 longations of the sheath, and on this view the sheath de- 

 scends actually to the basal membrane. The radii are 

 narrow : their edges have simple septa, with the inter- 

 spaces filled up solidly. The alae have their sutural edges 

 smooth. 



The Basis is membranous, and more or less convex. 

 This convexity is rendered possible by the support afforded 



