122 PROVISIONAL AID TO THE STUDY OF TASMANIA!* M0LLI7SCA. 



16. Coronate, the upper part of the 

 whorls having a series of revolving 

 tubercles or spines 



17. Varicose, when the external thick- 



ening of the outer lip of aperture 

 occuring in some shells during 

 rest-periods is not absorbed when 

 growth is resumed, but remains 

 crossing the whorls, rib-like, at 

 regular intervals 



XI, Operculum. A horny or shelly lid 

 which closes the aperture of a large 

 number of univalve shells. It is . — 



1. Concentric when nucleus is central 



or sub-central. 



2. Imbricated or lamellar when 



nucleus is marginal. 



3. Claw-shaped or unguiculate, 

 nucleus apical or in front. 



4. Spiral, revolving gradually and 



growing on one edge. 



5. Pauci-spiral, few rapidly increasing 



whorls. 



6. Sub-spiral, scarcely spiral. 



7. Multi-spiral, many whorled. 



8. Articulated when it has a 



projection. 



9. Radiated, modification of the 



articulated form in which the 

 spiral is not so evident. 



Example fig. 

 No. 



BIVALVES. 



XII. 



Arcuated, bent in the form of an arch. 

 Auriform, ear-shaped. 

 Auricled, having appendages like ears. 

 Byssus, a beard. 

 G-ibbous, bulging, swollen. 

 Compressed, squeezed together. 

 Concamerated, arched over, vaulted. 

 Concave, hollowed out 

 Concentrically rayed or ribbed, curved, 



with the umboe as a centre. 

 Cordate, cordiform, heart-shaped. 

 Crenulated, notched at the margin, 



scalloped 



(53) 



(53) 



