1849.] CARPENTER ON THE STRUCTURE OF ORBTTOLITES. 31 



Quoy and Gaimard ; and wliich has been recently brought again 

 under the notice of naturalists by Mr. Jukes, who collected it in 

 considerable numbers. These two agree closely in every particular 

 save the form of the superficial cells, which in the O. complanata 

 are nearly round (fig. 30), whilst in the iVustralian specimen they 

 are oval, or rather almost quadrangular (fig. 29) ; they must un- 

 questionably be regarded, therefore, as nearly allied species of the 

 same genus ; so that, if the O. complanata is to be regarded as the 

 fossil type of the genus, the Australian species must be considered as 

 the recent, and the generic term Marginopora must be dropped. In 

 both we find that the cells of the surface are closed, unless laid open 

 by abrasion, and that the only real apertures exist at the margin ; 

 so that the designation Marginopora is really as applicable to the 

 one as to the other. 



The structure of the Australian disk, as shown by a thin section 

 parallel to its surface, is delineated in fig. 24, in which the cells are 

 seen to be arranged with great regularity in concentric rows. It is 

 only near the margin of this section (as at a, a), however, that its 

 plane has passed sufficiently near to the surface, to show the cells in 

 the oval form which they there present ; elsewhere the section has 

 traversed a deeper stratum of the coral, in which the cells present a 

 rounded section when cut across. This difference in the form of the 

 cells as they pass down obliquely into the polypidom, is shown on a 

 larger scale in figs. 25 and 26 ; of which the former represents the 

 oval cells of the surface, laid open by a section passing through their 

 plane, and the latter a corresponding section of the round cells or 

 passages of the interior of the polypidom. Fig. 23 is a section of 

 O. complanata taken in the same direction as fig. 26, and exhibiting 

 a slight difference in the arrangement of the partitions, which causes 

 the entire disk to present some resemblance, when thus examined, 

 to the back of an engine-turned watch. 



The structure of the interior of the disk, as shown by a vertical 

 section, is exhibited in fig. 27 ; in which we see the oval cells of the 

 two surfaces, «, a, divided from each other by regular partitions, and 

 covered-in above by complete opercula ; whilst in the intervening- 

 portion of the disk, we see nothing but a series of round apertures 

 that seem less regular in their arrangement. If such a section be 

 made close to the margin, we lose the superficial cells, but we bring 

 into view the openings of the deeper rounded cells upon the margin 

 itself, as shown in fig. 28. I am not altogether confident in the 

 correctness of my interpretation of these appearances, which are 

 faithfully represented in the adjoining figures ; but I am disposed to 

 believe that if this disk was really formed by an animal or collection 

 of animals of the Bryozoal type, the round passages opening at the 

 margin, and penetrating obliquely into the polypidom, so as to be 

 cut across both in horizontal and vertical sections, constitute the real 

 habitation of the animal ; and that the ovate cells, which form so pe- 

 culiar a layer upon the surface, are a later production, not improbably 

 for the reception of ova. These ovate cells would seem to communi- 

 cate with the cylindrical passages beneath, by means of two small 

 apertures in each cell, as shown in fig. 25. 



