GRABAU : MONILOPORIDAE. 
419 
Almost from the beginning, as far as known, the characteristic 
ridge or carina appears along the center of the upper portion of the 
procumbent tube. It is probable, that in the very young initial 
corallite this carina is wanting, especially if this young individual 
was attached to some foreign object of support. On this point, 
however, observations are entirely wanting. The aperture of the 
immature tube is oblique, the lower wall of the tube projecting 
farthest forward, and ending in a gently rounded anterior lip. The 
lines of growth indicate this to be true of very young individuals, 
and it is also seen in the distal end of the last formed immature 
corallites (PI. 4, figs. 2-5). The same character is shown in adult 
specimens by the lines of growth, which form a sharp reentrant over 
the carina, pass obliquely forward over the sides, and describe a 
gentle forward curve on the flat face. This manner of growth indi¬ 
cates that the immature polyp bent upwards at the oral end after 
the manner of some modern Bryozoa and sedentary worms, a habit 
which became permanent in the adult individual, resulting in the 
upward building of the final portion of the tube. The resulting 
crowding of the poly]) against the upper edge of the orifice of the 
procumbent tube may have been a cause in the formation of the 
carina. The supposition, that the pressure of the polyp against the 
upper frontal edge of the corallite is responsible for the carina, seems 
to be borne out by the fact, that on the upturned calyx the carina 
gradually dies away, so that at the rim of the ordinary calyx there 
remains scarcely a notch to mark its position. This would be ex¬ 
plained by the gradual diminution of the pressure, which finally 
ceased after the calyx had reached a certain height. The fact, that 
away from the margin, where the pressure no longer existed, the 
internal section of the tube became circular through the addition of 
layers of sclerenchyma, also indicates that the marginal pressure had 
a close connection with the formation of the carina. yin important 
factor in the formation of the carina, was undoubtedly the friction 
of the polyp against the upper edge of the procumbent tube during 
contraction and expansion, and the crowding on the withdrawal of 
the polyp into the tube. In this respect the carina may be 
compared to the plications and groovings on the columellar lip of 
gastropod shells (see Dali., Trans. Wagner free inst., vol. 3). 
When the polyp has reached maturity, and gives off its buds, a 
broadening of the anterior portion of the tube occurs. This is fol¬ 
lowed by an extension of the lateral angles and an upward building 
