REPORT ON THE HEXACTINELLIDA. 489 



exhibits a beautiful Lyssacine structure. The same transition is exhibited by Rhahdo- 

 dictyum and Rhabdostauridium." 



" At any rate the Lyssacina were once the sole representatives of the Hexactinellida. 

 As a Lyssacine every Dictyonal form must still begin its development, though this stage 

 may only perhaps last for a very short time. And thus there was at any stage the 

 possibility of the Dictyoninal form becoming again reduced to a Lyssacine. The stifFest 

 and most brittle Dictyoninal framework differs after all only in degree from that of the 

 loosest of the Lyssacina." 



My own researches have convinced me that there is indeed a certain antithesis 

 between Lyssacina and Dictyonina, which may be justly recognised in the distinction 

 of these two systematic divisions of the order Hexactinellida, but that this difference is 

 not fundamental, nor involving the separate origin of the two divisions, but that it is 

 rather one of degree and in no way suggesting a hard and fast separation. 



In distinguishing the two groups, Zittel laid stress, however, not merely on the 

 simple fact whether the principal spicules were or were not united into a firm connected 

 framework, but rather on the manner in which this union was effected. It must be 

 allowed that what is regarded as the characteristic dictyonal mode of framework 

 formation, viz., by close apposition of the two corresponding arms of adjacent hexradiate 

 spicules, and the formation of a common sheath, is indeed very frequent, and in some 

 Dictyonina, like Aulocystis, or in the youngest portions of Farrea stocks is even normal 

 or perhaps constant, but it cannot be overlooked that it is extremely common to find that 

 the mode of union of the dictyonalia is in part, or here and there throughout, essentially 

 different. Zittel indeed eaUed attention to the fact that beside the hexradiate spicules 

 united as above indicated there were others " which left the series and had their rays 

 soldered arbitrarily to the rest of the framework. When one or two rays of such 

 irregularly disposed spicules become united by chance to the thickened centre of a 

 hexact, the result is obviously the apparent origin of more than six rays from one centre 

 of intersection. Other irregularities may also arise by the curvature or direction of 

 individual rays, so that two rays in one axis come to lie no longer in a straight line." 

 If these deviations, which may be readily detected in most Dictyonina, are really only 

 exceptions to the tj^pical mode of union which Zittel supposed to obtain, then the sharp 

 and certain separation of the Dictyonina from the Lyssacina with connected framework 

 cannot be said to be seriously affected. But there are, besides, not a few Hexactinellida in 

 which the supposed typical mode of union is either not to be detected at all, or only here 

 and there after careful search, and even then in a fashion so far from characteristic that 

 among the countless deviating and arbitrary intersections of the majority of the rays the 

 exceptional occurrence of the typical mode may be indeed regarded as a matter of 

 chance. Thus, for example, in the different species of the genus Aphrocallistes known to 



