118 FAMILY MILIOLIDA. 



to give place to the latter, whenever the chamberlets of the superficial layers are perfectly 

 separated from those of the intermediate stratum, and are connected only with the annular 

 passages. 



174. The "intermediate stratum," again, may be altogether wanting, notwithstanding 

 that the two superficial layers are separated from each other by a horizontal partition. In 

 this case, each layer may have its own annular canal, or there may be but a single canal, 

 which is then genei'ally very large ; and the chamberlets of the two layers have an alternating 

 arrangement as regards each other. Such an arrangement may present itself as one of the 

 modes of transition from the Simple to the Complex type, the cylindrical chamberlets 

 being disposed to subdivide transversely when they attain a considerable length, and the 

 annular canal to become double ; whilst in zones more distant from the centre, the two 

 layers are separated by the interposition of the intermediate stratum. Sometimes, however 

 the disk continues to increase and attains a considerable size on this duplex type, as we see 

 especially in the Orbitolites of the Red Sea ; and its edge then presents two rows of rounded 

 prominences with pores between them, those of the upper and lower rows alternating with 

 each other. It is on a dislc of this type that Prof. Ehrenberg has founded his genus 

 Amphisorus (xxxix), which I cannot regard as even specifically distinct from the ordinary 

 Orbitolik's. — There is sometimes a complete absence of regularity in the disposition of the 

 cylindrical chamberlets of the intermediate stratum, so that they present an assemblage of 

 indefinitely-shaped passages, communicating with each other in various directions. This 

 variety is chiefly interesting, as showing how little importance is to be attached to smaller 

 deviations of the same kind. Further, the septa dividing the contiguous chamberlets of the 

 same zone are occasionally deficient, so that the interior of the zone is a continuous circular 

 gallery, with only slight indications of the normal divisions ; thus corresponding exactly with 

 the peneropliform condition of Orhicidina (^ 143). In such a case, it is obvious that the 

 ring of sarcode must have been everywhere of nearly uniform thickness, showing no division 

 either into horizontal or into vertical segments; and it may not be thought improbable that 

 this is its first condition in every case, and that its segmental division is a subsequent process, 

 so that the shelly investment, if formed previously to the segmentation, will have the 

 character of incompleteness just described. I cannot lielp suspecting, that the peculiar 

 groove around the margin of the Fiji specimens formerly noticed (Ij 164), is referable to a 

 still greater incompleteness of the production of the calcareous investment around the newly- 

 formine zone. 



'& 



175. Reparation of Injuries. — Much light is thrown on the physiology of the Orbitolite 

 type of structure, by the examination of specimens — of which I have met with several — 

 whose conformation makes it evident that, after larger or smaller portions of the disk had 

 been broken away, a new growth has taken place along the fractured edge. Two examples 

 of this are shown in Plate IV, figs. 26 and 27. — In the first of these, it is obvious that so 

 large a portion of the disk has been broken away, as to leave only an irregular fragment, 

 including its centre and about an eighth of its margin. Here seven zones have been formed 

 since the injury ; and the chamberlets of these, whilst produced conformably to those of the 

 uninjured margin a, a, present the most marked want of conformity to those of the fractured 



