ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
131 
Plate IV.—Fig. 11. Tubular external layer of Lithodomns. Magnified 
500 diam. (§ 69).—Fig. 12. Flattened polygonal cells of internal layer 
^lymorpha ; these cells cut transversely at a, and obliquely 
at b. Magnified 25 diam. (§ 69).—Fig. 13. Section of middle layer 
of Chaim., showing cellular structure cut transversely at a, obliquely at 
i.i.aodlongitudinaJly at e, c. iMagrjified25 diam. (§ 71).—Fig. I-I. Tu¬ 
bular layer of ^rca wirbnia, uear the edge of the shell) showing that 
die tubali there occupy the spaces between the ridges, and are directed 
towards them. Magnified .50 diam. (§ 73).—Fig. 15. Section of ex¬ 
ternal layer of Ttuculu tnargarifactny showing the tubuli opening (as 
black points) upon the surface of the ribs ua,bh, whilst iu the clear 
spaces between them indications of cell-structurc are seen. Magnified 
100 diam. (§ 73). 
Plate ^ .—Fig. 16 . Section of extemai layer of Telli/M, parallel to the sur¬ 
face, showing Ipge fusifonn cells cut nearly in the direction of their 
length. Magnified 40 diam. (§ 76).— Fig. 17« Section of internal 
layer of Tellina, parallel to the surface, showing smaller fusiform cells 
cot somewhat obliquely. Magnified 80 diam. (§ 76).—Fig. 18. Ex¬ 
ternal celUayer of Mactra stuUorum. Magnified 80 diam. (§ 77).— 
Rg. 19. Internal cell-layei' of Lutraria elliptiea. Magnified 150 diam. 
(J 77).—Fig. 20. Cellular structure from oiiddle layer of Mya armaria. 
Magnified 150 diam. (§ 78). — F'ig. 21. Portion of the jmme layer, in 
which the boundaries of the cells are less distinct. MairiiiHed 150 diam. 
(§78). 
Plate VIr—Fig. 22. Another portion of the shell of Mya armaria, nearer 
the external surface, in winch the cells have distinct boundaries, and 
exhibit large dark nueleur spots. Magnified 150 diam. (§ 78).— 
Fig.23. Portion of the same layer, in which the cell-bouitduries ore ob- 
hterated, but ibe nuclear spots remuiii. Magnified 1.50 diam. (§ 78).— 
Fjg. 24. Portion of the same layer, in which the cells appear to have 
^leseed io sinuous rows. Magnified ISO diam. (§ 78).—Fig. 25. 
bxterual surface of the same sliell, aht>wing incomplete layer of large 
dispersed over a .-iulijacent couirihite layer of smaller cells, 
hlagnificd 150 diam. (§ 78). — Fig. 26. Large cells, with raiiiuting ar- 
of their calcareous contents, from section of tooth of Mya 
artnaria. Magnified SO diam. (§ 78)—Fig.27. Large cells of siinpler 
aspect, from the same tooth, passing on one siile Itjto the preceding, and 
p homogeneous substance. Magnified 80 diam. (§ 78). 
. p-'Fjg- 28. Elongated eells, from the tooth of Mya arenariuy 
irided oblhpiely, with eqimre tenninations. Magnified 80 diam. (§ 78). 
^Fig. 29. Fusiform cells, from the some tooth, divided nearly longi- 
Qiltnally, marked by trarwversc strite. Magnified 80 drain. (§ 78).— 
I Cellular layer of Carhula (vnicata ; the cells rounded and iso- 
ated in one stratum, polygonal and united in another. Magnified 80 
.*^■(§79).—Fig.3J. Superficial layer of Thrucia ro7/fw»6 showing 
isolated cells superposetl upon the proper external layer. Mag- 
™ 81)-—F'ig. 32. Section through the external laver 
' jhraaa dittoria, parallel to the surface, showing its very definite 
CL’lls with nuclear spots. Magnified ^ diam. 
\s ol).—Fig, 38. External layer of Myodoru striafa, showing large 
J*n^tic cells cropping out obliquely on the surface. Magnified 40 diam. 
p 
iayer o( Anafuui, showing distinct cells, of 
nich the boundaries are not sharply defined, and the contents have a 
K ^ 
