20 THE VOYAGE OF H.M.S. CHALLENGEE. 



ambiguous nature, since systematically the horny sponges present a kind of circtdus 

 vitiosus. Through Luffaria and Verongia, Euspongia is connected with Aplysina, but, 

 on the other hand, it is also, through Carteriospongia, connected with Spongelia; now 

 Spongelia through Velinea is connected with Ajjlysilla, and again A2)lysiUa through 

 lanthella is connected with Aplysina. It is evident that under such circumstances the 

 families of Dr. Vosmaer are by no means natural systematic groups. With regard to the 

 mutual relations of the genera of different families, such a circulus vitiosus is inconceivable, 

 and proves only that such families are not natural. As I remarked before, I adopt the 

 arrangement proposed by Dr. Vosmaer ; to attempt a better one would, I believe, be at 

 present premature ; but I adopt it only as an arrangement of a provisional character. 



The foregoing observations were begun in order to decide the question — Which 

 of the existing systems of the Keratosa is the most to be recommended ? The unexpected 

 result at which we arrived is of such high importance that its further foundation becomes 

 desirable, and as this latter demands a detailed discussion of characters used by classifiers 

 as systematic distinctions between different representatives of our conjectural families, I 

 now pass on to those of generic value. 



