REPORT ON SPIRULA. 
13 
median sagittal plane, but it is necessary to notice that this rolling-up is ventral or 
“ endogastric.” 1 The greater part of the shell is covered by the mantle, except 
at the two orifices indicated above. In Spirula reticulata (Fig. J) the shell extends 
further forward, under the integuments, than in Spirula peronii (PI. III.), the three 
last septa being hidden. 
The shell is entirely covered by a periostraeum continuous over the whole surface. 
The external face of the shell is not smooth, as in the majority of internal shells, but 
irregularly reticulated (Fig. K) in Spirula peronii, Spirula reticulata, and in all the dry 
shells I have been able to examine (in the mesh of this reticulum is another slighter 
reticulation formed by a fine broken line). On the last whorls this reticulation is not 
recognisable except upon a little more than the half of the convex face. On the concave 
face, especially in the first portions of the spire (PI. II. fig. 7), there is a median external 
band of thickening. 
The septa have a curved surface, with the convexity directed towards the summit of 
the spire (PI. II. figs. 5, 6). Their structure shows that they are attached to the 
internal face of the shell by a thin margin directed forwards. They are therefore distinct 
formations, but it is scarcely probable that they are much more recent than the portions 
of the shell to which they are immediately adjacent, for in the specimen of Spirula 
reticulata studied, the last chamber was, so to speak, without depth (see Fig. J, vi) ; the 
septum was then formed near the free margin of the shell. At the level of the first 
septum a constriction is observed (PI. II. fig. 7), less and less marked further on, 
which leads one to suppose that during the first stages of its existence the successive 
openings of the shell were somewhat narrowed. 
The initial chamber is spherical with a larger diameter than those which immediately 
follow. 2 This “ apical chamber ” is the true primitive embryonic shell; it follows 
from its form that the young Spirula had (as is likewise observed in the earliest stages 
of Dibranchiates) a spheroidal visceral mass (episoma) with a complete external shell (see 
Fig. D). The form and the relations of the shell to the mantle must only have been 
changed later on, with growth, after the manner indicated by the figures D to F above. 
The siphuncle is situated on the concave side (“ ventral ” for the later chambers) of the 
shell, and attached to the wall of that side (PI. II. fig. 7). The various segments 
(portions comprised between two successive septa), especially in the last whorls, are 
inflated in the half turned towards the opening (PI. II. fig. 5 ; PI. Y. fig. 1). Each 
segment is directly continuous with the septum which is “ anterior ” to it (that is to 
say, formed after it) or situated nearer the opening (PI. II. fig. 5). The septa do 
1 The other Molluscs with rolled-up univalve shells present, when they have not undergone torsion, a 
dorsal or “ exogastric ” rolling up, e.g., adult Nautilus, embryonic Patella and Fissurella. 
2 Certain authors call it “ ovisac ” ; it exists not only in other Dibranchiates (Belemnitidse, &c.), but also 
in the Ammonites. 
