of the Shell of Operculina Arabica. 171 



sent at these parts, particularly around the central cell, where 

 they assume the form of rays. 



Besides these tubes, a vertical section of the shell presents a 

 series of horizontal lines 7-8 or more in number, parallel to each 

 other, but not equidistant (fig. 3 c, c) ; these appear to be the 

 lines of contact of the layers of which the shell is composed. 



Lastly we come to the margin, which exhibits a very curious 

 and interesting structure. It is almost entirely composed of 

 calcareous spicula, arranged parallel to, but overlapping each 

 other (fig. 4). These spicula are l-237th inch long, and l-900th 

 of an inch broad, transparent, apparently hollow, and pointed at 

 each extremity ; they appear to be straight, although from 

 their position one would be inclined to think that they must 

 be a little curved. When a transverse section is made of the 

 margin, we observe that it consists of upwards of 100 of these 

 spicula, which form a triangular bundle or cord (fig. 6 a), the 

 apex of which is directed inwards or towards the chambers, and 

 the base («) outwards to form the free, rounded margin of the 

 shell ; while its sides are overlapped by an extension of the walls 

 of the chambers, which open as it were to receive it. Its base 

 presents a regularly wavy outline (when viewed m the trans- 

 verse section) from the longitudinal arrangement of the spicula, 

 which do not appear to be covered by a layer of the shell ; and 

 parallel to its sides run the papillary tubes of the chambers (Z>), 

 becoming more vertical as they increase in distance from this 

 position ; while towards its apex appear the divided large vessels 

 of the marginal plexus (c) . In the transverse section also, when 

 reduced to a thin layer, transparent intervals appear in the form 

 of zigzag lines radiating from the apex to the circumference of 

 the cord, which would seem to indicate that the spicula were 

 arranged in it in more or less horizontal planes, dipping towards 

 the apex. 



It will naturally now be asked, how this spicular cord (fig. 5 

 h, h), which commences with the first cell, terminates; but I 

 regret that there is not a single specimen in my possession to 

 afford the information. This arises probably from the extreme 

 thinness of the last-formed chamber ; for with the two or three 

 preceding ones, it is almost always broken or absent. All I can 

 state in connection with this is, that there ai*e always more or 

 less vessels of the marginal plexus cut through or broken in a 

 transverse section or fracture of the spicular cord, and frequently 

 a large one close to its apex, which, after the shell has been 

 filled with a solution of carmine and then laid in pure water, 

 purges it almost completely from the colouring matter with 

 which it had been filled ; — a broken interseptal vessel will also do 

 this. Hence it is not impossible, that a natural opening of the 



