REPORT ON THE GENUS ORBITOLITES. 17 



arrangement of concentiic annuli crossed by straight radiating lines (compare PI. I. fig. 1 

 with PL VI. fig. 4), their thickness does not exceed that of the smallest specimens of 

 Orbitolites marginalis, with which this type corresponds in the simplicity of its structure 

 (indicated by the singleness of the row of pores along its margin), but from which it is 

 obviously differentiated by the shape of its chamberlets, indicated by its surface-markings. 

 The disks of this species, which are usually remarkable for their flatness and regularity, 

 seem to attain a diameter of at least 0-6 inch ; but specimens of that size are seldom or 

 never brought up entire, their extreme tenuity, and the slight adhesion of their successive 

 annuli to each other, rendering them extremely fragile. Their thickness does not exceed 

 ■g^th of an inch. Th'e inner margin of each shelly annulus is slightly grooved, as shown 

 in PL I. fig. 4 ; and the two edges of this groove embrace the thin edge of the preced- 

 ing annulus, as shown in sectional view at a, a, fig. 3. In all but the central portion of 

 these disks (PI. I. fig. 1), the annuli are complete and of nearly uniform breadth : but 

 the inner portion of the disk shows a marked excentricity, the " nucleus " being con- 

 siderably out of centre, and the first-formed zones being developed from one side of it 

 only ; so that it is not until after repeated additions, that the cyclical plan of growth 

 characteristic of the Orbitoline type comes to be established. When the cavity of the 

 disk is laid open, either by grinding or by the action of dilute acid on its thin superficial 

 lameUse, or even when an unaltered specimen mounted in Canada balsam is viewed 

 by transmitted light, the radiating lines with which the surface is marked are seen to 

 correspond with internal partitions (PL I. fig. 2, and PL II. fig. 5), which divide each 

 flattened annular chamber into a multitude of narrow chamberlets. This division, 

 however, is not complete ; for the radial partitions do not extend to the outer margin 

 of the annulus, so that a sort of gallery is left, into which every one of the chamberlets 

 opens at its outer end. The septum which forms the peripheral wall of this gallery is 

 perforated by pores at regular intervals ; and each of these opens into a chamberlet of 

 the next annulus, — those of the outermost annulus opening along the margin of the disk 

 (PL I. fig. 5). It is characteristic of this species that the pores are more or less 

 elongated in the plane of the disk, instead of being either circular or vertically-oval, as 

 they are in other Orbitolites. Similar pores are seen on the internal (fig. 4) as well as the 

 external margin of any zone that has been detached by fracture ; and it is obvious that 

 they constitute the channels of communication between the central and peripheral portions 

 of the cavitary system ; whilst the annular galleries, seen in transverse section at b, b, b, 

 fig. 3, maintain the like continuity between the different portions of each zone. Thus, 

 whatever may be the number of these concentric annuli, a perfectly free communication 

 exists throughout ; the departure shown in this species from the general plan of structure 

 already described, having reference only to the shape of the chamberlets, and their relation 

 to the undivided gaUery. And it is at once seen that this departure marks out Orbitolites 

 tenuissima as an earlier and less specialised form; since if the chambers of a Peneroplis 



(ZOOL. CHALL. EXP. — PART XXI. — 1883.) X. 3 



