REPRODUCTION 95 
but in Rimula (Plate VII, Fig. 7) the anterior portion 
of the slit is bridged across with shelly matter when 
the animal is half grown, and the quondam slit 
remains as a perforation half-way between the apex 
and margin of the shell. In Fisswrella the same 
thing takes place quite early in the animal’s history, 
and the resulting perforation is left almost at the 
apex of the shell (Plate XXIX., Fig. 14). The series 
of holes in the shell of the Ormer (Haliotis, Plate 
VIL, Fig. 8) have a similar origin, the perforations 
resulting from a periodic bridging of the marginal 
slit during the creature’s growth. In Schizodentalium 
plurifissuratum, on the other hand, the perforations 
have been subsequently drilled, for there are more 
slits in the adult than in the young shell, and it is 
the habit of the Scaphopoda to absorb the apex of 
the shell in proportion as the aperture is added to. 
The very young shell of Dentalium (Plate XXIX., 
Fig. 6) is so deeply cleft as to be almost bivalve. 
A somewhat analogous parallel to Fissurella is 
offered in the common Bivalve Anomia. The adult 
shell is attached to various objects by the strong 
shelly byssus which passes through a circular notch 
near the umbo of the flat valve. When quite young, 
the little shell has no such opening, but after it has 
attached itself and begins to increase in size, the 
margin of the flat valve literally grows round the 
byssus, leaving the characteristic orifice, which, as 
the shell enlarges while it remains stationary, appears 
