OF THE GENUS MICROCIONA. O 



occasionally send ont a branch. The basal and dermal membranes 

 are alike in character, being thin and pellucid, both having a network 

 of fine cylindrical spicula, the terminations acutely pointed, and with 

 retentive spicula of the bihamate form, contort and sigmoid, of 

 various sizes, though the smaller must, I think, be regarded as 

 immature in growth. Besides these, are others of the anchorate 

 description, though less in number, yet occasionally abundant. The 

 larger kind is tridentate, palmate, equi-anch orate, with a few biden- 

 tate equi-anchorate. Of this latter is a smaller variety, more slender 

 in character, having the teeth projected outwards somewhat like the 

 barb of an arrow. 



The interstitial membranes are also provided with the two classes 

 of retentive spicula, at times in considerable quantities, often seen 

 attaching themselves to the larger spicula of the skeleton, showing 

 their office to be that of binding the membranes to it as well as 

 together, thus warranting the term " retentive," which Dr. Bower- 

 1 bank has given to them. 



The columns of the skeleton are composed of acuate spicula, 

 more or less spinous, but, like many of the genus, being more pro- 

 nounced at the base. At the apices they often project through the 

 membrane, giving a hispid appearance to the surface when the 

 sponge is dry, but not visible in its living condition. Spicula of a 

 similar character also project at various angles from the columns. 

 Besides these there are smaller spicula attenuato-acuate entirely 

 spined. 



In calling attention to the analogies with known species, mention 

 has already been made of 31. plumosa in relation to the size of the 

 skeleton columns. The spicula are very similar in character to 

 many of the genus, but those of the skeleton are mostly like to those 

 in M. Jict/t/a, differing, however, from all in having no inflation at 

 the base. The tridentate-palmate form in the membranes resembles 

 that in M. ambigua, and the angulate-bidentate is found in no less 

 than seven out of the twelve species already described. The 

 bihamate forms, which are found alone in this species of the genus 

 are sometimes so abundant on the dermal membrane as to appear 

 closely matted together, especially between the intervals of the 

 apices of the skeleton columns, and are of all sizes, some exceedingly 

 minute, but undoubtedly these must be considered as immature, as 

 before stated. A close examination of the membranes will amply 

 repay the observer in minute structure. There is evidently order in 



