GENUS HETEROSTEGINA: ORGANIZATION. 563 



which I understand to be very commonly met-with in Malta in fissures of the rocks, 

 but of which the age is uncertain. 



113. Organization. The older specimens of Heterostegina (Plate XXX. fig. 2) pre- 

 sent a form which, when regular, may be characterized as discoidal. There is, how- 

 ever, a knobby elevation or nucleus, which is usually somewhat excentric ; and from 

 this the turns of a spire are seen to commence, the last of which usually becomes con- 

 tinuous with one part of the margin of the disk (a b c), which there possesses a thick 

 and defined border. As this spire opens-out, however, it becomes thinner and flatter; 

 and this thinning is especially noticeable at that part of the margin of the disk (a dc) 

 which corresponds with the opening of the spire. An examination of this portion of 

 the disk shows that it precisely corresponds in structure with Cycloclypeus ; the form 

 and disposition of the chambers, their mode of communication, the structure of their 

 shelly walls, and the interposition of the intermediate skeleton and of its canal-system, 

 being all points of such close resemblance, that, as there is positively no other point 

 of difference than a somewhat inferior thickness of the intermediate skeleton between 

 the successive rows of chambers in Heterostegina, a fragment of this marginal por- 

 tion of the spirally-formed disk of Heterostegina might be taken for a fragment of 

 the cyclical disk of Cycloclypeus, without the possibility of certainly distinguishing 

 them, and vice versd. It is interesting to observe, moreover, how close is the con- 

 formity of these two types, even as regards their irregularities ; for it will be seen, 

 on an inspection of the figure, how little uniformity there is in the breadth of the 

 successive rows of chambers, and how frequently it happens that a row is incomplete, 

 just as in Cycloclypeus (^[ 96). 



1 14. If, now, we examine the structure and arrangement of that spirally-coiled por- 

 tion of the disk, which constitutes its nucleus, and which is best shown in younger 

 specimens, we see that, as in the other cases, the first chamber (Plate XXXI. fig. 1, a) 

 is globular, that the second (b) buds-forth from one side of this, and each successive 

 chamber from the outer side of the preceding, just as in Nummulite or any other 

 simple helical form. But before one turn of the spire is completed, each newly- 

 formed chamber is seen to be double (cc) instead of single, a small portion being 

 divided-off (as it were) near the marginal part of the whorl ; and just about the part 

 where the second turn is completed, the gradual opening- out of the spire gives room 

 for the interposition of a third chamber in each ro.v (d) ; and the number is soon 

 further augmented, in accordance with the progressive increase in the breadth of the 

 spire, the dimensions of the individual chambers retaining a pretty close conformity to 

 a constant average. An examination of this figure will further show, that the increase 

 in the number of chambers in successive rows always takes-place at the inner margin 

 of the spire; some of those nearest the outer margin dying-out, as it were, without 

 giving origin to new chambers in the next row. This may, I think, be connected 

 with the fact, that there is always a large free opening (e, e) between one row of 

 chambers and the next, at the inner margin of each spire (the situation of the open- 



MDCCCLVI. 4 E 



