EEPORT ON THE HEXACTINELLIDA. 283 



project more or less beyond it, and fill up to a greater or less extent the four angles of 

 the cruciate tangential rays of the pentact. Contrary to Carter's report, according to 

 which six clavulaj usually accompany each pentact, I generally found eight round each 

 in the best preserved specimens, and disposed so that two lay in each of the angles of the 

 tangential rays. This number does not seem, however, to be in any way constant. Not 

 unfrequently I have found ten or more clavulaj in one bundle, and very frequently fewer 

 than eight, and sometimes very few. It is difficult to determine whether they may be 

 sometimes wholly absent; where this appears to be the case, it is quite conceivable that 

 they may have fallen out or have been torn away. Nor does the fact that they frequently 

 project more or less beyond the surface of the skin necessarily imply that they have this 

 position in the living sponge. On the contrary, in the best preserved specimens 

 the normal position seems to be that in which the hemispherical terminal discs occur 

 within the dermal membrane. Deformation of the body-waU or pressure of apposed 

 foreign bodies has forced them outwards, as may be inferred indeed from their 

 shape. 



The form of the dermal clavulse in Farrea occa varies considerably, as Carter has 

 already noted, and that between somewhat wide limits, from the long-toothed umbels to 

 the smooth club (PL LXXII. fig. 8), nevertheless one definite form is so predominantly 

 frequent, that it may be regarded as the normal type, of which all the others are only 

 exceptional modifications (PL LXXI. figs. 5, 9). This typical dermal clavula exhibits a 

 club-shaped or poppy -head-Like terminal swelling, which is externally and laterally 

 roofed over by a hemispherically arched umbel, frec^uently raised in the centre into a 

 projecting boss. The overhanging margin of the umbel is toothed. The teeth, which 

 number eighteen to twenty-two are either simple marginal teeth resulting between 

 notches of the margin, or are more or less sharply defined off from the edge and laterally 

 compressed even at the base. The cylindrical stalk is gradually narrowed towards the 

 lower, simply rounded extremity, and is usually straight or gently bent, and, like the 

 poppy-head-like extremity, is beset with small irregularly disposed tubercles, which 

 increase somewhat in size and number towards the narrowed lower end (PL LXXI. 

 fig. 5). 



Various deviations from the above described typical form often occur both in different 

 regions of the same sponge and in specimens obtained from difi"erent localities. The 

 variation is especially associated with the terminal portion which we have shortly 

 designated the " head." In thickness and length, for instance, it varies greatly. In many 

 cases the stalk shows towards its upper end at most a gentle thickening, which bears 

 terminally a thin toothed umbel like that of a toad-stool, while in others there is a broad 

 club-shaped terminal portion from which the umbel region is but slightly, or even not 

 at all marked off", so that a perfectly smooth club-like swelling may result. While the 

 terminal portion of the umbel is sometimes quite flat and without any central elevation 



