PLATE IV. 



Fig. 



1. Vertical section of the perforated valve of Waldhelmia {Terehratuld) australis, through 



the median plane, from the beak at a to the margin at b ; showing that it mostly 

 consists of but one layer, everywhere traversed by vertical passages. At a a, 

 h b, and c c, are indications of the successive marginal additions which the shell 

 has received. The layer, separated by the Hne d d, has the appearance of bemg 

 a production subsequent to the general enlargement of the shell, apparently for 

 the purpose of strengthening its older and thinner portion. — Magnified 6 

 diameters. 



2. Portion of vertical section of the loop-bearing valve of Terehratella ricbicunda, taken 



in the transverse direction through the origin of the calcareous appendages, 

 a a. The higher magnifying power enables the vertical passages to be more 

 distinctly seen ; and it is shown that the calcareous appendages are not traversed 

 by them. — Magnified 18 diameters. 



3. Portion of fig. 1, near the extremity a, considerably enlarged, so as to show the 



ordinary trumpet-like form of the vertical passages, and the remarkable con- 

 traction which they exhibit in the inner and (probably) later-formed layer. 

 The line dd, corresponds with the same line in fig. 1. — Magnified 100 diameters. 



4. Another portion of fig. 1, at the part c, considerably enlarged, showing the occasional 



bifurcation of the vertical passages. Magnified ] 00 diameters. 



5. Fragments of the shell of Kraussia rubra, separated by spontaneous disintegration. — 



Magnified 100 diameters. 



6. Thin lamella of shell of Waldheimia (Ter.) australis, showing the parallel disposition 



of the flattened prisms, and the regular arrangement of the passages which 

 intervene between them ; and at a the outcrop of these prisms on the internal 

 surface of the shell. — Magnified 100 diameters. 



