312 Mr. H. J. Carter on the Structure of the Foraminifera. 



On the other hand, the more important fact, which Dr. Car- 

 penter has failed to recognize after my description of it, is the 

 spicular structure of what he terms the " marginal cord," which 

 had been previously named by myself the " spicular cord," to 

 denote its peculiar composition, and under which appellation, 

 although I like the former name much, I must still continue to 

 allude to it. 



I need hardly quote all that Dr. Carpenter has stated respect- 

 ing the structure of this part of the test of Opcrculina ; suffice it 

 therefore to notice that he considers its structure homogeneous, 

 and not spicular. Thus he observes, " the supposed spicular 

 composition of this ^marginal cord' (as it may be appropriately 

 termed) is due to the peculiar manner in which the homogeneous 

 substance of which it is composed is traversed by the set of canals 

 that are correctly described by Mr. Carter as forming the * mar- 

 ginal plexus *.' " 



Now, I would rather not have had to repeat what I stated 

 respecting the structure of this cord ten years since ; and I feel 

 certain that, if Dr. Carpenter had taken a favourable specimen of 

 Operculina, and with a small, sharp scalpel had cut off tangcn- 

 tially portions of this cord, there would have been no occasion 

 for it. However, these subjects seldom lose by a accond inves- 

 tigation, and the result of mine in this instance is, that I am 

 not only more convinced of the spicular structure of the cord 

 than ever, but am now able to describe its composition much 

 more definitely than has hitherto been done. 



Thus, this cord, which is almost semicircular, with the arch or 

 convex part outwards and the chord or base within, is composed 

 of — 1st, spicules; 2nd, an interspicular substance; and, 3rd, a 

 plexus of anastomosing canals (PI. XVII. fig. 11). 



The spicules (fig. 10) are semitransparent, homogeneous, cry- 

 stalline, calcareous bodies, more or less fusiform in shape, and 

 arranged one above another irregularly and intcrdigitatingly, in 

 horizontally inclined planes, which, like the leaves of a book 

 when open, radiate partly from the centre of the " chord " and 

 partly along this chord on either side — that is to say, they do 

 not all radiate from a common point; while there are also a 

 certain number of semiplanes at the circumference, which fill up 

 the intervals formed by the radiating of the whole ones (fig. lie). 



The interspicular substance is an areolar, calcareous, membra- 

 nous structure, which unites the spicules together and the planes 

 to each other respectively. It resembles and is analogous to the 

 albuminous tissue which surrounds and unites the spicules in 

 the spiculiferous Sponges. 



Lastly, the plexus of anastomosing canals consists of reticu- 

 * Phil. Trans. 1859, p. 25. 



