BY E. F. HALLMAXK. 457 



treinities of the (radial) fibres, the terminal spicules of which 

 show no tendency to spread penicillately. The outlines of the 

 fibres, as seen in cross-section, are very irrei;ular (PI. xxvi., fi,>;.0). 

 In the axial re,i,'ion of the skeleton, the fibre-spicules are less 

 compactly and less regularly arranged than in tlie radial fibres, 

 and the appearance of irregularity is much increased by the 

 presence of many additional spicules lying between the fibres; 

 outside the axial region, interstitial megascleres are exceedingly 

 rare. The characteristic microscleres — the spinispirult« — are 

 scattered always in great abundance throughout all parts of the 

 interior, and at the surface occur closely crowded in a welldetined 

 layer, which constitutes the dermal skeleton. The microstrongyla, 

 when present, are confined to the extra-axial choanosome. 



The chief specific differences, in so far as structural features of 

 the skeleton are concerned, are with respect to: — (i.) the dejisity 

 of the axial fune; (ii.)the ratio between the diameter of the fune 

 and that of the whole branch; (iii.) the stoutness of the skeletal 

 fibres: (iv.) the amount of spongin entering into the composition 

 of the fibres; [v.) the frequency of connection between the radial 

 fibres by means of transverse fibres; and (\i.) the angle of in- 

 clination oi the radial fibres, i.e., their direction relatively to the 

 longitudinal axis of the branch. A further difi'ei'ence, ho\ve\er, 

 is presented by T. pi(.'<tii/(isnf<, in which the skeleton is axially 

 condensed only in the stalk and in the lowermost porti(jns of the 

 branches: while in T. bacteruim, apparently, an axial condensation 

 is not developed. In order most readily to perceive, and also 

 most accurately to determine, the distinctive characters of the 

 skeleton in the different species, it is necessary to study the 

 skeleton freed of the soft parts. 



The megascleres are slightly curved oxea and strongyla (and 

 I'are styli), occurring intermingled, and coiniected by intermediate 

 forms; the oxea, on the average, are slightly longer and stouter 

 than the strongyla and not so nearly of uniform diameter, but 

 otherwise differ from them only in the character of their ex- 

 tremities. Both in regard to the sha})e and the size of the 

 megascleres, the two species differing most widely are T. scabrutsas 



