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 Fissarella 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 

 VII., 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 



