GENERAL FEATURES. 95 
The Monactinellidx are represented by four genera, Reniera, Avinella, Atracto- 
sella, 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 Feniera, several species of which appear to be present. 
The pin-shaped spicules referred to Avinella 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 Paleozoic Tetractinellide belong to the widely distributed genera, (eodites 
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, Geodites deformis, from the Carboniferous of Ayrshire, is remarkable 
for the relatively large size of the spicules. 
The Lithistidee are very sparingly represented in our Paleozoic 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, Aulocopiwm, 
Oswald, very abundant in the Baltic basin, is entirely absent in this country. 
Hexactinellid Sponges are by far the most numerous in the Palzozoic 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 fenestrata, 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 Paleozoic 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 
