26 



Guide to the Invertelrata. 



Ascoceras. perforated by a siphuncle which is somewhat inflated between 



GALLEKY the septa ; the upper and larger portion contained the animal. 



The whole shell is usually slightly curved, its aperture being 



case 7. simple. Yery fine examples of this genus may be seen in 



Table-case Fig. 47. — A. Schematic view of the interior of Ascoceras manubrium, Lindstr. 



57. 



Upper Ludlow (U. Silurian), I. of Gothland, showing the structure and 

 arrangement of the septa : si, siphuncle ; dt, duct that communicates with 

 the siphuncle of the Nautiloid portion of the shell («, fig. F). B. Schematic 

 view of three sigmoid septa of Ascoceras fistula, Lindstr., seen from the 

 ventral side. C. View of the third septum of the same species, shown as 

 free, as if detached from the shell, to exhibit the large central lacuna. D. The 

 same, viewed laterally (the siphuncular orifice is seen at the bottom of all 

 these figures). E. Longitudinal section of a specimen of Asc. decipiens, 

 Lindstr. , from Sandarfve kulle (hill) , with four regular septa above the sigmoid 

 ones. F. Schematic view of Asc. decipiens, represented as if complete ; 

 n, the Nautiloid portion of the shell. G. Longitudinal and median section of 

 the posterior part of the shell of Choanoceras mtdabile, Lindstr., showing 

 the interior, with the outlines of the incomplete septa ; si, siphuncle. 

 H. Exterior of the same specimen, reduced to about one-third natural size. 

 A model of Ascoceras (executed by Mr. G. C. Crick. F.G.S.) is placed in 

 Table-case 57 at entrance to the Cephalopod Gallery (Gallery VII). 



Wall-case 7, from the Carboniferous Limestone of Ireland. It 

 must have attained considerable dimensions, for an example of 

 Poteriocerm cordiforme from the Red Sandstone of Closeburn, 

 Dumfriesshire, is nine inches long and seven and a half ipches 

 in its greatest diameter. 



