130 
REPOBT— 1847 * 
126. My distance from the coast has hitherto deprived me of the oriw- 
tunity of examining a Crab or Lobster in the act of fornuDg a new shell aftii 
the exuviation of the old one ; and I cannot therefore speak with anv certtinij 
in regard to the mode in whicli the dernml layer is developed and caicifisl 
There can be little room for doubt, how«!vcr, that the process of the fornuiM 
of this layer of tJic testa is precisely analogous to the development oT ih 
dentinal structure ; and that it originates, like the latter, in a cellular 
covering (in this case) the entire surface of the true skin. The panlb 
lines brought into view in a vertical section can scarcely be anythiug el* 
than indications of successive stages of calcification; and the objectioo it 
such a view, drawn by M. Lavalln from the fact that there is a ptffo 
continuity of structure between the successive bauds, ceases to have Uj 
weight, when it is rcmemberetl that the same continui^ exists in deotiifc ^ 
which is unquestionably calcified in succeasive layer# from without iiis'afi ' 
List of Illustrations. 
Plate I.— Fig. 1. External surface of 7erel>ratu/a (rustra/ist ns ^ 
transmitted light wlien the shell has beeti reduced in thickness from ih 
inner side; showing the perforations in the shell covered-in by met* 
branous discs. Magnified dO diam. (§ 58 ),—Fig. 2 . External mea- 
brane (epidermis ?) of the same .shell, separable by maceration in 
acid; showing its oontiimity, with an increase of thickness, over^ ' 
orifices of the iKirforatious. Magnified- 1-0 diam. (§ 58).—Eig-f ^ / 
calcified nienibrane of shell of Ta'uhratula caput Kerptnlia \ sliyffisg*^ ' 
caecal tubes which occupy the perforations, and their connection by la 
baseruentMimmbranc of the shell: 3, «; another portion, showing 
opercular discs attuched to the caocal terminations of the tubes. 
nifted 150 diam. 58).—fig. 4-. Cmcal tubulus from the shell of 
aitstmlis, showing the contained cells: 4, small portion of the p^J" 
tie of the same shell, on which Ito numerous minute cells, con«p|'‘’| 
ing m size and appearance with those contained within the ceecal hiWi 
Magnified 150 diam. (§ 5B). , 
Plate H. fig. 5. Vertical .section of decalcified membrane of 
which has spimtaueously split into laminm of variable thickfl»^ 
alway«, howt-ver, along the course of the transverse striff- f 
seen the edges of two laminaj, each of which seems to consist o 
a single layer of flattened cells; and at h, b are seen the surf^; 
...vn.l... I - ... ’ »r -jjlQjJXI 
Plate 
ephxpptuH. Magnified 250 diam. (§ 60). . 
TE Ill.— lug. 8. Vertical section of die shell of VniooeddmM^. 
a ime pa^nig from tlie mnl>o to the lip; a a', b b', c c, weU-tnafJ/f 
of growtlu evidently indicaUng distinct formaiioos; the 
between these seem to indicate si.cces*;ive stages of the 
w^failsi the lines Mow c c' arc probabl v the Uoumkries of 
^thmlammm formed after the shoUhas attained nearly its 
Magnified 5 diam. (§ 6S).-Fig, 9. Portion of the preccd.og ^ 
transveiwe atriation of tlie 
S , ^ (nearc) enlarged to show the suct-essiou « ‘JJ, 
intervening epidermis, in the tbicUflea '■ 
Magnified 40 diam. (§ 63). ^ 
