GENERAL FEATURES. 95 



The Monactinellidse are represented by four genera, Eeniera, Axinella, Atracto- 



i, and Haplistion. Entire Sponges and fragments of the connected skeleton 

 of Haplistion have been preserved, but the other genera are only represented 

 by detached spicules. These, however, so closely resemble the skeletal-spicules 

 of existing species of these genera, that the generic affinity of the ancient with 

 the recent forms can hardly be doubted. With the exception of the peculiar 

 spicules from the Silurian referred to Atractosella, monactinellid Sponges in our 

 area do not appear in force until reaching the Carboniferous. Some of the cherty 

 Sponge-beds in the Yoredale series of Yorkshire and North Wales, and at a 

 corresponding horizon in Sligo, Ireland, mainly consist of the minute, detached, 

 cylindrical spicules of Beniera, several species of which appear to be present. 

 The pin-shaped spicules referred to Axinella are comparatively rare, and only 

 occur in the Carboniferous ; and Haplistion is likewise limited to the same forma- 

 tion in the West of Scotland. 



The Palaeozoic Tetractinellidae belong to the widely distributed genera, Oeodites 

 and Pachastrella. They are as yet only known by detached acerate, trifid, and 

 four-rayed spicules, and by the reniform spicules of the dermal layer of the former 

 genus. Their first appearance is in the Carboniferous, and they play an important 

 part in building up some of the Sponge-beds of the Yoredale series in Yorkshire. 

 One species, Oeodites deformis, from the Carboniferous of Ayrshire, is remarkable 

 for the relatively large size of the spicules. 



The Lithistidse are very sparingly represented in our Palaeozoic rocks by four 

 genera ; of these Cnemidiastrum and Doryderma are only known by detached 

 spicules. Only a single example of Astylospongia has been discovered, and in this 

 the spicular structure has been obliterated so that its identification is not 

 altogether certain. The genus is of such common occurrence in the Silurian 

 strata of North America and the Isle of Gothland, that its comparative absence in 

 the British area is worthy of note. The genus Hindia, again, which has a similar 

 wide distribution, is only known in this country by a single fragment lately 

 discovered in the Ordovician strata of Ayrshire, and some detached spicules 

 in the Carboniferous Rocks. Another well-marked Silurian genus, Aidocopium, 

 Oswald, very abundant in the Baltic basin, is entirely absent in this country. 



Hexactinellid Sponges are by far the most numerous in the Palaeozoic strata. 

 Twelve genera belonging to this sub-order are known, some, however, only from 

 detached spicules or small fragments of the connected skeleton. The earliest 

 known fossil Sponge, Protospongia fenesirata, Salter, from the Menevian beds of 

 South Wales, is included in this sub-order. The genus Hyalostelia is the most 

 widely distributed ; it is present in every division of the Palgeozoic Rocks, except 

 the Devonian. The Sponges of this genus, like the existing Hyalonema, were 

 furnished with bundles or wisps of elongated rod-like spicules which served to 



