XAUTILID^. 
75 
Remarl s. A more detailed description of the peculiar diaphragm 
mentioned in the above diagnosis may here he given. (See fig. 9.) 
It may be considered, says Barrande, as a true septum, similar to 
the one which forms the base of the body-chamber, but with its 
concave side directed inwards, and so characteristic is this position 
of the diaphragm that if an isolated body-chamber is found, it is 
Fig. 9. 
Hercoceras minim. — w, shell-wall ; d, diaphragm ; s, siphuncle. Copied from 
plate ccxli. fig. 3, of Barrande’s ‘ Sjsteme Silurien clu Centre cle la Boh^me,’ 
vol. ii. pt. i. 18GG. Keduced from the original figure about one half. 
difficult to distinguish the anterior from the posterior extremity. 
To compensate for this closing of the extremity of the spire, there is 
a deep emargination, occupying the whole breadth of the convex 
wall of the shell. This aperture is of trapezoidal form, the upper side 
of the trapezium corresponding with the border of the diaphragm. 
The other two sides slope obliquely inwards. (See fig. 8, 6.) Upon 
each of the lateral borders of the orifice there is a more or less 
deep lobe, varying in form and position. Thus, the lateral border 
may be merely interrupted by a wide and shallow sinus near its 
posterior extremity, or there may be, instead, a deep lobe at the 
lateral angles. (See Barr. Syst. Sil. vol. ii. pi. cii. ff. 4, 6, 7, 8.) 
These lobes recall those observed in the large orifice of Phrag- 
moceras. Barrande observes that the varying development of these 
lobes may be the result of the state of preservation of the individual 
observed, or their relative age and the degree of absorption that the 
test has undergone. In fact it is found that the test composing the 
body-chamber and the diaphragm, instead of being a waU simple in 
