170 



PORIFERA. 



posed cross-shaped spicules of smaller size (fig. 50). Owing to the 

 condition of preservation of the sponge, it is still not absolutely 

 certain whether the spicules were fused with one another or free, 

 but it would seem probable that the former was the case, and that 

 the genus belongs therefore to the Dictyonine section of the 

 Hexactinellids. 



Another very ancient genus of this order is Hyalostelia, one of 

 the Lyssakine section of the Hexactinellids, which is first known 



in the Cambrian deposits, 

 is found in Ordovician and 

 Silurian strata, and is abund- 

 antly represented in the Car- 

 boniferous rocks. Detached 

 spicules from the Chalk 

 have also been referred 

 here. The genus Hyalos- 

 telia is related to the recent 

 genus Hyalonema, compris- 

 ing the so-called " Glass-rope 

 Sponges." The body of the 

 Sponge in Hyalostelia is im- 

 perfectly known, but is com- 

 posed of siliceous spicules 

 some of which are of the 

 hexactinellid type, with one 

 ray much elongated, while 

 others are variously modi- 

 fied. The sponge was at- 

 tached to the sea-bottom by 

 a long "rope" of cylindrical or rod -like, siliceous "anchoring 

 spicules," which often terminate at their distal ends in four recurved 

 rays (fig. 60). The " rope " is the part usually found in the fossil 

 condition, and in the case of the Hyalostelia parallela of the Car- 

 boniferous rocks it was originally described as a tubicolar Annelide, 

 under the name of Serpula parallela. 



In strata of Ordovician age, the Hexactinellid Sponges are rep- 

 resented not only by the genus Hyalostelia, but also by the singu- 

 lar group of the Receptaculitidce, the true relations of which have 

 been recently determined by Dr George J. Hinde. The two prin- 

 cipal genera of this family — viz., Receptaculites and Ischadites — have 

 long been known to palaeontologists, but the investigation of their 

 structure has proved to be attended with great difficulties, and 

 various opinions have been held as to their relations and systematic 

 position. In Receptaculites (fig. 61) the organism is a cup-shaped 

 or platter-shaped body which grows from a small inversely conical 



Fig. 60. — a, Part of the anchoring-rope ot Hyal- 

 ostelia Smithii, from the Carboniferous Limestone, 

 of the natural size ; b, Fragment of one of the anchor- 

 ing spicular rods of the same species, showing four 

 recurved rays at the distal end, enlarged ten diame- 

 ters. (After Hinde.) 



