1899.] Foord. — Carboniferous Brachiopoda and Mollusca. 75 



body-chamber. Through these chambers by means of a 

 central, or nearly central perforation there passes a delicate, 

 tubular, cord-like process, the siphon or siphuncle, which 

 proceeds from the dorsal end of the body. The functions of 

 this tube are unknown, but it is a distinguishing feature in all 

 Cephalopod shells. 



Thus is built up a structure which is matchless for strength 

 and lightness and beauty of form. L,et us close this descrip- 

 tion of the shell of the Nautilus with the well-known lines of 

 the poet-philosopher : — 



" Year after year beheld the silent toil 

 That spread his lustrous coil ; 

 Still as the spiral grew 

 He left the last year's dwelling for the new, 

 Stole with soft step its shining archway through, 

 Built up its idle door 



Stretched in his last-found home, and knew the old no 

 more." 



The siphuncle is very large in some of the straight-shelled 

 Cephalopods of Palaeozoic times, as in Acti?ioceras, a gigantic 

 species of which is found in the Carboniferous Limestone of 

 Ireland, the largest coming from Castle Espie (Comber), 

 County of Down. 



The coiled shells of the fossil Cephalopods, though varying 

 greatly in detail, are constructed upon the same general plan as 

 the shell of Nautilus. In the Goniatites, another extinct type 

 of Cephalopods, having close-coiled shells, the edges of the 

 septa, as seen in casts from which the shell is removed, are of 

 a zig-zag form and a very definite pattern which has been 

 relied upon in dividing this group into genera and other 

 larger divisions. 



And now a few words as to those internal organs not yet 

 dealt with. 



In most of the Mollusca (except the Iyamellibranchs) there 

 is on the floor of the mouth an organ called the radula or 

 lingual ribbon, on which are rows of innumerable, delicate, 

 chitinous teeth whose points project backwards, its function 

 being to break up the food as it passes over it into the gullet. 

 The vascular system is generally well developed, although the 

 blood flows partly through spaces among the organs. The 

 heart consists of one, or of two auricles (in Nautilus four, 



