Chap. VI. POCILLOPORA DAMICORNIS. 295 



a curve, following an upward or a downward course, that is, trending toward the 

 apex, or base of the stem ; and occasionally a cell has a double curve, in a plane 

 parallel to the axis, and also laterally {Ficf. 13). The perpendicular curvature 

 accounts for the fact, that in breaking a stem, the surface of fracture is usually 

 curved, the stem naturally giving way along the line of least resistance. 



A structure like this does not occur among the Corals of the class of Polyps ; 

 it is peculiar to the Tabulata, as a comparison with those of Pocillopora and 

 Seriatopora, described in the following sections, unquestionably shoAvs. I am, there- 

 fore, satisfied that the whole group of Tabidata must be referred to the class of 

 Acalephs, in which they find naturally their place, among the Hydroids. 



SECTION II 



POCILLOPOKA DAMICORNIS LMK. 



The Corallmi. — The youngest cells (PI. XV. Fig. 14, a) of the corallum are about 

 as strictly defined as the older onesj there is none of the uncertain, irregular 

 limitation between the outlines of the cells and the spaces intervening, as we 

 have observed in Millepora, but the corallum is deposited in a solid mass {Fig. 14", 

 c), close up to the boundaries of the pit.s. The intervals, at first, are very thin 

 and fragile, and crested by rather irregular, spiniform projections, which are arranged, 

 generally speaking, in a single row. Similar spines, but shorter and more conical, 

 are scattered all over the sides and bottom of the cells. In consequence of this 

 close proximity of the cells, they are necessarily polygonal, and usually five or 

 six-sided. After a cell has developed to a certain degree, and obtained a depth 

 equal to one third its width {Fig. 14", a h), it loses its simple character, and becomes 

 transversely partitioned, and, at the same time, excepting at the forks of the branches, 

 where the hydroids are crowded to a certain extent, changes its contour, and 

 becomes more circular; and, finally, the intervals widen, sometimes to such an 

 extent, that, in the oldest part of the stem, they are as broad as the cells between 

 which they lie. All the chambers which are shut off from the outermost or 

 youngest division of the cell, are perfectly smooth, the intervals between the spinules 

 being filled up by calcareous deposit, which, at the same time, thickens the inter- 

 vening walls of adjacent cells to a greater or less extent. The transverse dia- 

 phragmic partitions are quite firm, and sometimes of considerable thickness, and 

 are usually slightly arched across the cell, and imperforated {Fig. 14''). In the 

 oldest cells, three quarters of an inch deep, there are at least from thirty to 



