24 
NAUTlIiOlDEA, 
Eemarks. The truncation of the septa in certain species of fossil 
Cephalopods has been made the subject of exhaustive study by 
Barrande, by whom the phenomenon was first discovered in 1855 ' 
in Ascoceras, and subsequently in a communication made by him 
to the Geological Society of France ^ elaborately illustrated and 
described by means of the present species. 
Finding that the extremity of the shell in all individuals of 
0. truncatum, both old and young, had been covered by a new test, 
M. Barrande came to the conclusion that the septa must have been 
successively thrown off by the animal, and the broken end repaired 
regularly and periodically by a fresh shelly deposit. 
To explain this reparation of the truncated septa, M. Barrande 
assumed that the animal possessed long palmated brachial append- 
ages, more or less analogous to those of Argonauta^ and capable of 
reaching the basal portion of the shell, and endowed with a power 
of secretion of the same nature as that of the mantle, but more 
potent, reproducing exactly the external layer of the test. 
On examining the terminal septum in 0. truncatum we find that 
it is covered with calcareous deposits which consist of several 
layers, easily separable by percussion, the whole forming a sort of 
conical cap {calotte conique) to the shell, corresponding nearly to 
the shape of the septum it covers. The layers of the first deposit 
{depot conicpie) exhibit a groove or depression, more or less deep, 
proceeding from the periphery towards the siphuncle, and consti- 
tuting a median line, following the greatest diameter of the 
septum, and dividing it into two equal and symmetrical parts. 
The surface of each of the layers shows a series of irregular 
longitudinal striae converging towards the siphuncle, which latter is 
sometimes central, sometimes a little eccentric. 
Upon these layers a thin one is deposited, distinguished by the 
regular, longitudinal, fine, and close-set striae with which it is 
covered, These striae converge towards the siphuncle like those of 
the first deposit, but they are more regularly disposed. The outer- 
most layer is very thin, and instead of the striae taking a longi- 
tudinal direction, they are transverse or concentric. Their course is 
interrupted by a bending inwards towards the siphuncle, along a 
median line, corresponding in position with that of the groove 
found upon the first deposit {depot conique'). This bending inwards 
of the striae, which may be called a sinus, has its convexity turned 
Leonhard and Bronn’s ‘ Keues Jahrbuch fiir Min.,’ Jahrg. 1855, Heft iii. 
p. 280 . 
2 Loc. rif. 
