REPORT ON THE MONAXONIDA. 129 



schwachen Fiissen steht." Vosmaer, in 1885/ gave a better diagnosis: — " Stabnadoln 

 glatt oder gedornt ; letztere vorwiegend. Spongin nicht oder kaum vorhanden. Anker 

 drei-, selten zweizahnig, beide Enden gleich. Oberflacbe am lebenden Objekt scliliipfrig. 

 Canalsystem nacli dem dritten T}^us." This, however, is still very wide and might be 

 made to include a great variety of forms. The Challenger material has enabled us to 

 give a more definite diagnosis, taking as our starting point the original species, Myxilla 

 rosacea, Lbkhn. 



Still it may be questioned whether we have not made the genus too comprehensive, 

 in that we have admitted certain species possessed of echinating spicules.^ Vosmaer* 

 has already done this in admitting Bowerbank's Hijmeniacidon paupertas into the 

 genus, but without pointing out the great importance of the step which he has taken. 

 If we once admit forms with a distinct, special, echinating spicule, like that of Myxilla 

 paupertas, into the genus Myxilla, then we do away with all family distinction between 

 the Desmacidonidse and Ectyonida. This step appears to us to be very desirable, as the 

 distinction is a very unimportant one, and in the present work we have considered the 

 Ectyoninse (old Ectyonida) as a subfamily of the Desmacidonidse. 



The genus Myxilla as now constituted falls into two fairly natural groups ; (l) 

 species without definite echinating spicules, such as Myxilla rosacea, Lbkhn., and 

 Myxilla digitata, nobis ; (2) species with definite echinating spicules, such as Myxilla 

 paupertas, Bk., and Myxilla compressa, nobis. The detailed comparison of two such 

 forms as Myxilla rosacea, var. japonica, and Myxilla compressa furnishes a sufficient 

 justification for uniting both groups under the same generic name, at any rate for the 

 present. It wiU thus be readily seen that the genus really occupies an intermediate 

 position between the subfamilies Esperinfe and Ectyoninse. 



The genus Hastatus of Vosmaer comprises simply Myxillas of thoroughly normal type, 

 the point on which its author lays especial stress, viz., the hastate ends of the dermal 

 spicules, being well shown in the type species of Myxilla {Myxilla 1'osacea) ; and the 

 type species of Hastatus {Hastatus luridus) belongs to the same section of the genus as 

 Myxilla rosacea by virtue of its spined megasclera and the absence of echinating spicules. 

 The other species {Hastatus dichiei) assigned to the genus apparently agrees with Hastatus 

 luridus in all essential details. 



Our knowledge of the relations of Myxilla to other genera does not rest on a very 

 satisfactory basis (see under Plumohalichondria and Clathria, with which genera, as 

 well as — perhaps more distantly — with Hymedesmia and Hymerrhaphia, it presents 

 considerable affinities). 



' Bronn's Klass. n. Ordnung. des Thierreichs, Porifera, p. 349. 

 2 The " internal defensive spicula " of Bowerbank. 

 ' Notes Leyden Mus., vol. ii. p. 127. 



(ZOOL. CHALL. EXP. — PART LIX. — 1887.) ^^^ ^^ 



