FOSSILS— THEIR NATURE AND INTERPRETATION. IO5 



which, and the intimate structure of the shell substance, are 

 generally well preserved. Figures 2^^ 24. 



Fig. 23.— ^'olluscoida, Bryozoa. A , B, Fcnestelln. retiYortnis Sch\. Permian. ^ , a funnel-shaped 

 stock from the outside. A', enlarged view showing the cell mouths {o) and the perforations (/) 

 between the cell-rows of the coenoecium. C, Archimedes wortheni Hall. Carboniferous. 

 The stock consists of a broad coenoecium (hi), wound spirally about a central axis Ux\. Frag- 

 ments of the coenoecium separate from the axis present a structure similar to that of Fenes- 

 tella. 



Fig. 24 — Molhiscoida. Brachiopoda ,4, B, C, Inarticulata. i^tngula. A, Z, anatina Brug., 

 living, pedicle valve from within, st = pedicle ; .y, d, a, s\ muscular impressions. B, L. 

 tenui.Ksiiita Br. Triassic. C, L. Bcani Phill. Jurassic. Z?, E, Brachiopoda articulata. 

 Atrypa reticularis L. sp. Devonian. /), surface view of brachial valve. E, view of in- 

 terior, the brachial valve being in great part removed \/= foramen for passage of the pedicle; 

 cr = crura ; b — jugal processes or jugum ; s/> = spires or spiral coils of the brachidium. 



Mollusca. — Of the true Mollusks, all the four classes, 

 Lamellibranchiata, Gastropoda, Pteropoda, Cephalopoda, 

 construct, in most of their genera, calcareous or horny shells, 

 external or internal, which are preserved, more or less per- 

 fectly, in a fossil state. Gastropods and Lamellibranchiates 

 in the older rocks arc very apt to be in the condition of im- 

 pressions and moulds, the substance of the shell being dis- 

 solved and carried away ; this is also the case with many 



