96 BRITISH PALAEOZOIC SPONGES. 



attach the organism to the sea bottom. These bundles of anchoring spicules are 

 more frequently preserved than the six-rayed spicules of the body of the Sponge, 

 and they have been found in the Tremadoc rocks of Wales, and the Ordovician 

 of Girvan, Ayrshire ; but they are more particularly abundant in the Sponge-beds 

 of the Yoredale series in Yorkshire and in the decayed chert of the corresponding 

 rocks in the "West of Scotland and Ireland. Entire beds of rock in Yorkshire are 

 filled with them. 



The genus Dictyophyton is only represented by a small form, which appears to 

 be limited to Ludlow strata near Kendal, whilst it seems to be entirely absent 

 from the Devonian strata in this country. In North America, on the other hand, 

 and in Belgium, this genus has a great development in the Upper Devonian. 



Plectoderma is as yet only known by a fragment of the connected skeleton, it is 

 limited to the Upper Silurian of the Pentland Hills. Phormosella is likewise 

 restricted to a single horizon of the Silurian in Shropshire. Like many of the 

 Palaeozoic hexactinellids, it appears to have been a gregarious form. 



Whilst the spicular characters of many of the Palaeozoic hexactinellids correspond 

 closely with those of existing Sponges of this division, there are others in which 

 the spicules are greatly modified, and widely diverge from the normal type. Thus 

 in the Carboniferous genus Spiractinella, Hinde, the rays are ornamented with a 

 spiral ridge ; and, though simple six-rayed spicules occur, in the majority the rays 

 divide and subdivide, so that the extreme forms are stellate. In Holasterella, 

 Carter, stellate spicules are likewise present, and the larger spicules of the skeleton 

 also appear to be very irregular in form. Still more abnormal are the large 

 branched and spined spicules of Acanthactinella, Hinde, from the Carboniferous 

 of Ayrshire. In Amphispongia again, limited to a single horizon of Upper 

 Ludlow age, and to a single locality in the Pentland Hills, the spicular structure 

 strikingly differs from that of any other hexactineliid genus. In the well-marked 

 family of the Receptaculitidae, one ray of the normal hexactineliid spicule is modi- 

 fied into a delicate plate. 



Only detached spicules of the genus Astraospongia have as yet been met with 

 in the Silurian and Devonian strata of Shropshire and Devonshire, though in North 

 America, and in Germany, entire Sponges are occasionally preserved. 



The most striking of any of the Palaeozoic Sponges are the forms which I have 

 described under Tholiasterella and Aster actinella, and placed in a new sub-order, 

 the Heteractinellidae. Their spicules are of unusually large dimensions ; the 

 number of rays is variable, ranging from six to thirty, and they are disposed so as 

 to form either stellate or umbrella-shaped spicules. The spicules appear to have 

 been partially free and partially fused together in the skeleton. These remarkable 

 forms have as yet only been found in the Carboniferous Rocks of Ayrshire. 



