440 Mr. H. J. Carter on the Lithistidse, 



Short Commentary on the Lithistidse. 



The remarks regarding the microscopical examination of 

 the Hexactinellidse (see p. 363) apply to the Lithistidse ; but 

 while the two large specimens respectively of Dactylocalyx 

 pumiceiiSj Stutchbury, and MacAndrewia azorica, Gray, in 

 the British Museum, are very much alike in their dish-shaped, 

 wide, circular heads, each of which is supported by a thick 

 short stem, and their minute structure has the same vitrified 

 glassy appearance, it should be remembered that they form 

 representatives of two totally different systems of sponges, 

 the former being built upon a sexradiate division of the spi- 

 cule, and the latter upon a quadriradiate one ; that is, while 

 the Hexactinellidse have six ends to their spicule, the Lithis- 

 tidse have only four, and the lowest system of spicules in 

 which the linear form prevails, of course, only two. Hence 

 the spicule of the Hexactinellidse has, as it were, three shafts 

 joined together in the centre, thus giving six ends ; the next, 

 or quadriradiate system, has, as it were, two shafts joined 

 together in the form of a cross, thus presenting four ends ; 

 while the simplest system of all has only one shaft, and 

 consequently only two ends. Thus modified, forms of all the 

 three systems may be found in the Hexactinellidse ; but only 

 those of the third system with the Lithistidse — that is, no 

 sexradiate spicules. Of course I allude to the staple spicules 

 here, and not to the monstrosities which may occur in either 

 system ; and perhaps, too, I should restrict these observations 

 to the spicules of the skeleton-system, or skeleton-spicules, 

 since, when we come to the minute or flesh-spicules, we find 

 the multifid form of the sexradiate spicule, viz. that of the 

 rosettes, passing, as before stated, into the stellates and globu- 

 lar siliceous balls of the Geodinidse &c, and of course vice 

 versa. 



But, be this as it may, there seems to be very little doubt 

 that the system of the Lithistidse is that of Packastrella, 

 Geodia, and Stelletta, and never that of the Hexactinellidse : 

 that is, a sexradiate spicule, as just stated, is never found in 

 the Lithistidse. 



In Dactylocalyx Bowerbankii and D. Masoni the surface- 

 spicules are nail-like, having the spike or shaft directed 

 inwards and the three arms of the head bifurcated and spread 

 out horizontally, so as to meet those of the other surface- 

 spicules, while the interspaces are occupied by the sarcode 

 charged with the flesh-spicules, in the midst of which are the 

 pores ; and thus the dermal aspect is completed. In D. Masoni 

 the subspiral shaft of the minute or flesh-spicule is covered 



