GENUS ORBITOLITES. 115 



each other, seems indicated by the fact that there is no constant numerical relation between 

 them, — a circumstance which extremely perplexed me, until I had ascertained, by exami- 

 nation of the animal, that the passages proceeding from the former (Plate IX, fig. 8, in, ?«') 

 debouch, not (as I had at first supposed) into the columnar chamberlets c ,c', but into the 

 annular canals, //, U , which serve to bring the superficial and columnar segments of each 

 zone into mutual communication. 



167. As the description now given of the superficial layer applies equally to both 

 surfaces, we may now proceed to the intermediate stratum. When this is laid open by a 

 horizontal section (Plate IX, figs. 7, 9), it is seen to consist of a series of concentric zones, 

 the chamberlets of which alternate with each other, like those of the Simple type (^ 157). 

 The chamberlets are usually cylindrical (or nearly so) in form ; but often differ considerably 

 in size in different parts of the same disk, and sometimes even in different parts of the same 

 zone. It may be often observed that the cylindrical cavities do not always pass from end 

 to end in a straight line (Plate IX, fig. 11); nor do they always maintain a complete isolation 

 from each other, an inosculation of two columns (which is indicated in vertical sections 

 like that represented in Plate IX, fig. 8, by irregularly disposed apertures) not being unfre- 

 quent, while more rarely there is a fusion of two columns into one. All these features of 

 structure presented by the shell, are beautifully displayed by the animal (Plate IV, fig. 25) ; 

 the columns of sarcode c c, dc exhibiting the generally cylindrical form, the not unfrequent 

 inosculation, and the occasional fusion, which we have seen to exist in the cavities which 

 they occupy. At their upper and lower extremities, they unite with the annular stolons 

 JjJj , hU , which pass continuously round, in each zone, between the intermediate and the 

 superficial layers. 



168. Save in the case of such accidental inosculations as those just noticed, no other 

 lateral communication seems to exist between the contiguous chamberlets of the same zone, 

 than that which is established by the annular stolons just mentioned. The chamberlets of 

 the successive zones communicate with each other, however, as in the Simple type previously 

 described (^ 157), but with a curious modification; for whereas a horizontal section of the 

 latter shows that each chamberlet communicates with the two chamberlets alternating with 

 it in the interior zone (Plate IX, fig. 6), a hke section of the Complex type seems to show 

 that such a connexion exists with only one chamberlet of the interior zone, by a passage 

 running obliquely from one to the other, and extending continuously through several succes- 

 sive zones (Plate IX, figs. 7, 9, e, e), the very same section exhibiting opposite obliquities in 

 contiguous parts. The study of vertical sections, however, made tangentially instead of 

 radially, so as to cross these connecting passages, shows the explanation of tbis apparent 

 anomaly to be simply as follows. Each cylindrical chamberlet really communicates with 

 the two alternating chamberlets in the next interior zone, but by two distinct passages, 

 instead of by the divarication of one ; these inter-zonular passages are not upon the same 

 plane, but those of different planes are directed alternately towards one side and the other ; 

 and thus, as the disks are seldom perfectly fiat, the section which traverses, at one part of 

 the disk, the set of passages running in one direction, will traverse the other set of passages, 

 where, by the flexure of the disk, the plane of section is slightly altered in regard to it. So 



