ACTHSrOCEKATID^. 
195 
half their circumference, while in the other half they are flattened. 
A large funnel-shaped sheath, recalling the similar structures in the 
siphuncle of Pilocems, forms a conical chamber, which occupies the 
upper or more expanded part of the siphuncle. This sheath, doubt- 
less originally membranous, has a sinuous outline, due to the con- 
cavities of the siphuncular segments, which it enters for a short 
distance, the indentations between the segments (where the septa 
originate) causing a corresponding projection in the wall of the 
sheath. The apical extremity of the latter is often digitate. The 
whole of the apical portion of the siphuncle, as well as the space 
around the sheath, is inflltrated with calcareous matter, the cavity 
of the sheath being fllled with the matrix. Owing to the eccentric 
position of the siphuncle in the shell, its elements are somewhat 
oblique to the vertical axis. The septa are moderately approximate. 
The test, known only by a very small fragment, is extremely 
thin, and has a perfectly smooth surface. An endosiphon is present, 
but it is not very well preserved in any of the specimens 1 have 
examined. The largest example of Discosorus known to me is the 
one figured by Barrando {loc. cit. pi. cccclxxiv. ff. 7, 8), from the 
environs of Lake Michigan. This measures inches in length, 
1-J inch in its greatest, and 7 lines in its least, diameter. The 
largest specimen in the IN^ational Collection does not exceed inches 
in length. 
llemarhs. A good deal has been learnt concerning this singular 
genus since it was described by Hall (1851)\ whose knowledge of 
it was derived from incomplete specimens, which consisted only of 
the siphuncle, found in the Clinton Formation at Ontario and Lock- 
port, in the State of Hew York. Specimens of the siphuncle had 
been previously (1822) collected in the Niagara Formation of Drum- 
mond Island, Lake Huron, by Dr. Bigsby^, who briefly characterized 
them as columns usually tapering rapidly, composed of circular 
discs, with rounded edges, placed one upon the other.’’ 
Again, Prof. Hall (1852)’’ had occasion to deal with the present 
^ Foster and Whitney’s Report on the Geol. of the Lake Superior Land 
District, p. 222, pi. xxxiv. ff. 2, 3. 
^ Trans. Geol. Soc. 2nd ser. vol. i. 1824, p. 204, pi. xxx. ff. 3, 4, 6, 7 {not f. 5). 
^ Pal. of New York, loc. cit. I may here take occasion to observe that in 
tlie preface to this work the author states that though the printing of the 
volume was completed in the early part of the year 1850, it was not ready for 
publication until 1852. This explains how it is that one finds, under Hall’s 
description of Discosorus conoideus in Foster and Whitney’s Report (1851) a 
reference to the volume of the Palaeont. of New York, which was not published 
till 1852. 
o2 
