ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
133 
shown a small portion of its inner part more enlarged, in which is seen a 
very distinct cellular structure, ilagnified 450 diam. (§ 96). —Fig. 53- 
Section of the compact portion of the base of Calyptrwa, allowing lioly- 
gonal cells. Magnified 60 diam. (§ 97).—Fig. .54. Section of cancel¬ 
lated portion of base of Cnlyptraa, showing round diHcoldal cells over- 
lyinp one another obIir|udy. Magnified 60 diam. (§ 97). 
PwTE .\ni. — Fig. 55. Large flat cclU intervening between horny plates 
and nacreous lavers of ffnlioti-i sjjlendeim. Magnified 125 diam. (§ 98). 
—Fig. 56. Small cells of the nacre of the same shell, cut tiwnsversely 
at a, obliquely at 5, and showing their tt-ntiinations on the surface at c. 
Magnified 450 diam. (§ 9S).—Fig. 57. .Section of one of the calcareous 
plates in the tendinous gizzard of livltn., showing it? cellular structure. 
The boundaries of the cells clearly defined at a ; indistinct at b. 
Magnified200diani. (§ 99).—Fig. 58. Section of middle layer of shell 
of iVttitca, showing its cellular structure. Magnified !2.5diani.(§ 101). 
—Fig. 59. Section of dark outer layer of shell of Turbo pica, showing 
its cellular structure. Magnifinl 125 diam. (§101).—Fig. 60. Section of 
shelly operculum of Tui^o, showing the long prismatic cells, with well- 
marked transverse striae, of which it is composed. Magnified 125 diam. 
(§ 101 ). 
Plate XIV.—Fig. Gl. Section of the inner layer of the shell of l^chinus 
Uvidiis, showing the calcified areolar tissm 
fl. a, portions of a second reticulation seen 
first Magnified 164 diam. (§ 107).—Fig. 
shell of Erhinus^ showing the existence of the reticulated structure 
throughout, but its coai'ser character in the internal layer o. .Magnified 
102 diam. (§ 107).—Fig. 63. Longitudinal section of a small part of a 
spme of Echinus, showing the open areolar structure (o) passing into 
a solid plate (i), which is marked by the junctions of the connecting 
pillars (c). Magnified 860 diam. (§ 107).—Fig. 64. Caleai eou.? disc or 
skeleton of ambulacral tube of Echinus. Magnified 164 diam. (§ 111). 
—N.B. The figures in this Plate are taken from the illustration# to Prof. 
Valentin’s IMonograph on the Anatomy of thu germs Eckinnii, in the 
aeries of‘Monographics d’Echinodermes, Vivans ct Fossiles,’ par L. 
Agassiz. 
Plate XV.—Fig. 65. Tnvrisverse section of spine of Echinus lucunter ; 
<^ 0 ., internal row of pillars; b, b, succeeding row; c, c, external row of 
pillars; </,(/, interrupted portion; e,e, new luycr covering it. Magni¬ 
fied 14 diam. (§ 112).—Fig. 66. More enlarged portion of the central 
part of the .same spine, .showing its loose areolar structure. Magnified 
I50 diam. (§ 112).—Fig. 67. More enlarged portion of the external part 
of the same spine, showing its closer texture and solid pillars. Magni¬ 
fied 150 diam. (§ 112 ). ■ 
Plate XVI.—Fig. 68. Transverse section of spine of Echinus triffonarius. 
Magnified 18 diam. (§112).—Fig. 69. Portion of the same section more 
enlarged. Magnified 150 diam. (§ 112). 
late Xyil.—fig. 70 . Transverse section of a small spine of Echinta 
(»pecie8unknown), showing a remarkably large proportion of solid cal¬ 
careous substance. Magnified 45 diam. (§ 112 ).—Fig. 71. Enlarged 
portion of the central part of the same section ; a, a. inner circle of solid 
pulars. Magnified 1.50 diam. (§112).—Fig. 72. Enlaiged portion of 
outer part of the same section; a, a, outer row of solid pillar.?; b, b, 
pillars of the row witliin the preceding. Magnified 150 diam. (§ 112). 
^laie XVIlI._Fig. 73. Portion of longitudinal section of spine of Echinus 
I ot wnicn It IK coinposca; 
through the meshca of the 
62. 
