Fig. 



at a is seen a partial layer of prismatic substance arranged vertically, whicji was 

 probably the seat of the muscular attachment in the young shell ; at 5 a similar 

 layer, in the situation of the muscular attachment in the adult shell. — Magnified 

 40 diameters. 



9. Portion of the internal surface of the shell of Crania Norvegica, which affords the 

 muscular attachment, highly magnified, showing the extremities of the com- 

 ponent prisms, and the large orifices a a of the perforations. — Magnified 150 

 diameters. 



10. External surface of the shell of Thecidmm pumUa (Recent), showing large orifices of 



perforations arranged in rows. — Magnified 100 diameters. 



1 1 . Portion of decalcified membrane of Crania Norvegica, with its attached cseca, these 



sometimes bifurcated. — Magnified 100 diameters. 



1 2. External surface of the shell of Crania Norvegica, showing the radiating arrangement 

 of the subdivisions of its canals. — Magnified 100 diameters. 



13. Decalcified membrane of the prismatic portion of the shell of Crania Norvegica, 



showing dX a a the cells in their longitudinal aspect, and ?ii b b their truncated 

 extremities. — Magnified 150 diameters. 



14. Vertical section of shell of Strophomena aculeata, showing the shell a a to be traversed 



by perforations b b, round which the shell-structure is arranged in a funnel-like 

 manner ; c c, the matrix, the substance of which has penetrated the canals. — 

 Magnified 34 diameters. 



15. Section of shell of Strophomena aculeata, nearly parallel to the surface, showing the 



perforations divided transversely at a a, and obliquely at b b. — Magnified 34 

 diameters. 



16. Section of shell oi Discina lamellosa, showing its minutely tubular structure obliquely 



divided at « a, and transversely divided at b b, where the tubuli seem to lie in 

 clusters, presenting an appearance of cellular arrangement ; at c a more distinct 

 indication of cellular structure presents itself. — Magnified 150 diameters. 



