CHESTER — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 743 



thickn(\ss, the outer ones ;iro very Ions; iiiid li;iv(> imicli thicker \\;ills 

 iiiul thinner eavities. To th<' naked ey<" this yixcs the uppearaiiee 

 of 11 solid eortex. 



Where a hraneh is formed there is a l)reak between the inarruw of 

 the hraneli and that of the stem (Phite 4, I"'if;. (il), and the axis-i-ortex 

 forms a thick knee-like union at the stem for stren^tli. At the base 

 of the colony the axis-cortex is spread upon the sul)stratum and the 

 marrow is lacking;-. Many of the Bermuda colonies that were located 

 on the shallow ri'efs, where the tips were exposed at the lowest tides, 

 had lost the fleshy envelope of the tips of the hranehes for the distanee 

 of a few centimeters, the homy axis heinfj; here covered hy diatoms. 

 The fleshy coenenchyma was ,<,n'owin.i!,' loosely around the old axis and 

 taking the shape of the original branch. In such regenerated tips no 

 new marrow was formed. In a few instances the old axis had been 

 completely covered, but the coencsarc had not grown enough beyond 

 the axis to give evidence of the presence or absence of a nuirrow distal 

 to the old dead axis. 



The coenenchyma is composcnl of two regions, an inner, exhibiting 

 longitudinal canals, and an outer, containing the calycles or polxp- 

 chanibers. The longitudinal-canal region is characterized by a number 

 of large canals running parallel to the axis (Plate 1, Fig. 1, r(in. hi.), 

 and by the presence of purple, irregularlx' stellate spicules (Fig. B, 

 5-7) loosely massed in groups {spc.'). The spicules are represented 

 diagrammatically in the left half of Figure 1, to show their positions. 

 Tile longitudinal canals are less numerous at the tips of the branches — 

 where there may be eight to ten — than they are at the bases, where 

 twenty or more may be found. The diminution in number from the 

 base to the tips is flue to the running together of two adjacent canals 

 or to the abrupt ending of one or more. Some of them run continu- 

 ously from the basal half of the stem into the lower (abaxial) face of a 

 branch. On the axial face of the branch they may be continuous 

 w-ith those of the stem, or the canal may begin abruptly in the branch 

 without such connection. At the tips and on the branches the 

 longitudinal canals connect with one another and with the polyp 

 cavities by smaller canals (can.). At tlic base of the colony they run 

 out radially and end blindly, or are connected with one another or 

 with polyps by the smaller canals. At the tips the purple spicules 

 are loosely arranged in two concentric cylinders separated from each 

 other by the zone of canals. Where the branches are older, however, 

 the inner cylinder breaks up into small groups, which lie between the 

 axis and two adjacent canals (Fig. 1, spc.'); here the spicules interlock 



