OF THE StONGIADiE. 221 



of the character and purpose of the spiculum on which 

 they are based. 



The range of the stellate spicula are very considerable. 

 They are found abundantly and constantly in Geodia, 

 Packymatisma, Tethea, Dactyloealyw, and AlcyonceUum, and 

 in some species of ■Sponpi^, Mciyocylindn-us, and other 

 genera. 



11. l!he Oearia and Gemmules. 



Where the ovaria exist they afford exeell^t descriptive 

 characters^ Their construction is the same throughout the 

 whole of the known species of Geodia and Paxihymatisma. 

 The varieties in their form, although not always easy of 

 description, are yet readily distinguishable by a practised 

 eye ; and the difference in the degree of stoutness of the 

 radiating spicula of which they are constructed, and the con- 

 sequent fineness or coarseness of the reticulations on their 

 surface, very often affords good discriminative characters. 



In SpongUla, the varieties in their shape, and the 

 strikingly distinct forms of their component spicula, render 

 them exceedingly efficient for specific descriptions ; and 

 without them it wotdd, in several instances among the 

 exotic specieS) be very difficult to find descriptive characters 

 to separate one species from another. 



Excepting in Diplodemia, where the structural peculi- 

 arities of the ovarium are widely different from the preced- 

 ing instances, we know very Kttle more of these organs ; 

 but there is good reason to believe, from certain forms of 

 spicula detected in the deep-sea soundings, the sources of 

 which are at present unknown, that other marine spoiages 

 possess ovaria with which we are at present unacquainted. 



The gemmules afford very efficient specific characters in 

 some species of Tethea; but in the greater number of 

 Halichondroid genera, althou^ frequently present in abim- 

 dance, they agree so closely in structure with each other as 

 to render them of very little use as specific eharacters. 



We thus find that we possess eleven distinct varieties ©f 



