48 BALANID.E. 



They are solid, that is, they are never permeated by pores ; 

 but their edges are generally crenated, and there is, in 

 some cases, as in Chelonobia, sufficient evidence that these 

 crenations answer to the horizontal septa on the edges 

 of the radii (also often reduced to mere crenations), and 

 consequently, likewise, to the longitudinal septa of the pa- 

 rietes. In Coronula the edge of each ala consists of a 

 medial ridge, sending off denticulated septa on both sides, 

 and is therefore anomalous as compared with the alse in 

 other genera, but corresponds in structure with the simi- 

 larly anomalous radius of Coronula. In order to allow of 

 the growth of the edge of the ala, a fine thread of corium runs 

 up the narrow furrow in which the edge is lodged, proceed- 

 ing from the corium of the sack. In proportion as this 

 thread runs up higher or lower, so are the summits of the 

 alae rendered, during growth, less or more oblique. 



Sheath. — As the compartments overlap each other, the 

 edges of the alae would have projected, and the inner sur- 

 face of the orifice of the shell would not have been smooth 

 and rounded, had not that part of each wall, which does 

 not overlie an ala, been thickened so as to allow of the 

 formation of a shoulder or indentation, against which the 

 edge of the ala fits and abuts. The thickened portions, and 

 the alae themselves, together form the sheath, of which the use 

 seems to be to strengthen, like a broad internal hoop, the 

 upper part of the shell round the orifice, where naturally it 

 is weak. The sheath is composed of successive, fine, shelly 

 layers, which extend, as the shell is added to at the 

 basal margin, lower and lower down on the inner surface 

 of the walls. The lower edge of the sheath either simply 

 projects a little inwards, or more commonly is formed into a 

 sharp depending ridge, as represented in fig. 1, k', PL 25. In 

 some species of Pyrgoma (PI. 13, fig. 2 b), the sheath reaches 

 nearly to the bases of the compartments ; and in Chelonobia 

 (PI. 14, fig. 4 ece), the inner layer of shell surrounding the 

 sack, which seems to correspond more nearly to the sheath 

 than to the inner lamina of the walls, actually rests on the 

 basal membrane. The opercular membrane is generally, 

 but not invariably, attached only a little way above the lower 

 edge of the sheath : at each exuviation, a new opercular 



