GENUS OPERCULINA, 259 



Plate XVII, fig. 2). It will be seen from these delineations, all of which arc faithfully copied 

 from specimens in my possession, that the distribution of the interseptal canals, whilst presenting 

 a certain general uniformity of plan, is by no means constant in detail. In Figs. XLII, a, and 

 XLIII, D, we see two principal trunks passing from the two angles of the fissure at the interior 

 edge of each septum to join the marginal plexus at its exterior (as shown in Plate XVII, fig. 4), 

 and sending out branches which ramify over the part of the septum that joins the spiral 

 lamina (Fig. XLIII, d), leaving the intervening portion untraversed. But in Fig. XLII, a 

 (as in Plate XVII, fig. 2), we observe two or more such trunks proceeding from each angle 

 of the fissure, those of the same side frequently inosculating with each other; in Fig. 

 XLIII, c the two principal trunks on the opposite sides inosculate as they approach one 

 another in their course towards the outer margin of the septum ; and in Fig. XL this inos- 

 culation is seen to take place much nearer the inner margin of the septum of the last whorl, 

 so that the greater part of the septal plane is occupied by the plexus formed by their inter- 

 lacement. The interseptal canals of one septum are occasionally, if not invariably, connected 

 with those of another, by loops formed between the ramifications of those canals which 

 extend along the alar prolongations of the septa towards the centre of the shell. Moreover, 

 branches of the same system are generally found to penetrate the non-tubular shell-substance 

 whenever it accumulates in any quantity ; these branches are at once distinguished from the 

 ordinary tubuli (as in fig. 12, Plate XVII), by their much larger size, their diameter being 

 commonly about 1 -5000th of an inch. Those which proceed towards the spiral lamina 

 sometimes pass towards the surface in the non-tubular shell-substance that constitutes the 

 septal bands, as is shown in Fig. XLIII, d, and in Plate XVII, fig. 1 ; they are also frequently 

 seen to diverge from the septa over the walls of the chambers, — still, however being usually 

 invested by a layer of transparent shell-substance, as is shown in Plate XVII, fig. 4. This 

 last figure represents an appearance of areolation I have several times met with in the walls 

 of the chambers, — though usually less conspicuously than in this instance, — which has made 

 me suspect that the ramifications of the interseptal canals sometimes (if not as a regular rule) 

 form a network in the spiral lamina over the entire surface of the chambers. 



446. The interseptal system of each whorl is connected with the marginal plexus of the 

 preceding by a very remarkable arrangement. At each junction of the marginal cord with 

 the tubular substance of the spiral lamina, the orifice of a large canal (Plate XVII, fig. 2, 

 /.', //) may be seen in a vertical section which divides this canal transversely ; and either of 

 these canals (fig. 1 , //, h,) may be clearly traced in sections parallel to the surface that have 

 happened to pass through its plane, as at //, //, fig. 6, in which, by a fortunate accident, one 

 of the spiral canals has been laid open in its entire course through the inner whorls, and can 

 be traced nearly to the central chamber. In the tangential section, fig. 1 0, the two canals 

 n, a', can be seen running on the outside of what may be called the iiurJeus, consisting of the 

 central chamber and the chambers immediately surrounding it. The branching-off" of the suc- 

 cessive interseptal passages from these two spiral canals, is shown in fig. 6 ; and the relations 

 of the different parts of the system are brought extremely well into view in fig. 7, which 

 represents a tangential section of a young shell, the rapid curve of whose spire causes the planes 

 of even the contiguous septa to vary greatly in their direction. The spiral canals //, //, though 

 only running along the angles of the marginal cord, as shown in transverse section at fig. 5, //, li , 



