FOSSIL HEXACTINELLIDA 



207 



Fossil Hexactinellida. 



This group has the distinction of including among its 

 Lyssacine members the oldest 

 known sponge, Protospongia fenes- 

 trata, of Cambrian age (Salter). 

 As preserved it consists of a 

 single layer of quadriradiate, or 

 possibly quinqueradiate spicules, 

 which, arranged as a square 

 meshed lattice, supported the 

 superficial layer of the sponge 

 (Fig. 101). Whether or not the 

 fossil represents the whole of the 

 sponge-skeleton does not appear.^ 



The extraordinary Recepta- 

 culitidae are probably early 

 Lyssacine forms : they are cup- or 

 saucer-shaped fossils, abundant in Fig. 101.— Part of tlie specimei] of /"ro- 



,-, -1 . 1 , n ■ T^ • tospomiia fcnestrala in the Sedf^wick 



bilurian and above all m Devonian Museum. Cambridge. Nat. size, 

 strata, and have been " assigned (-^^'er Soiias.) 

 in turn to pine cones, Foramini- 



fera. Sponges, Corals, Cystideans," and 

 Tunicata. Hinde^ brings forward im- 

 portant arguments for retaining them 

 among Hexactinellida. The only elements 

 in the skeleton of the simpler genera, e.g. 

 Ischadites, are structures comparable to 

 \,^. .? Hexactinellid spicules. The sm-face of 

 ^~^^^ /'' "^-^' the fossil presents a series of lozenges 

 Fig. 102.— a portion of the forming a regular mosaic. Each lozenge 

 outer surface of a Recepta- jg the expanded end of one of the rays 



cuhtul, Acanthncoma bar- . . \ . in 



randei, in which the ex- ot a spicule ; it coiiceals four rays m one 

 panded outer rays of the i tangential to the wall of the cup- 



spicules are partially de- ^ ' o r 



stroyed, revealing the four shaped fossil, while the sixtli ray pro- 

 jects vertically to the wall into the cavity 

 of the cup. In the genus Becei'ftaculites 

 itself there is an inner layer of plates abutting against the inner 



' Sollas, Quart. Journ. Gcol. Sue. 1880, p. 362. 

 - Quart. Journ. Gcol. Soc. xl. 18S-i, p. 795. 





tangential ravs V>eneath. 

 3. (After Hinde.) 



