PACHASTRELLA. 153 
mainly of four-rayed spicules, mingled loosely together without definite arrange- 
ment; acerate spicules are also present. The rays of the tetractinellid spicules 
may be either simple or furcate, equal or unequal in length ; one ray may be deve- 
loped so as to form an approximate shaft, or it may be reduced to a blunted knob 
or even disappear altogether. In some instances also, one ray is prolonged beyond 
the central point of junction, so that the spicule becomes five-rayed. 
As the skeletal spicules in this genus are originally only held together by the 
soft perishable spongin, entire Sponges are of rare occurrence in the fossil state, 
and they have as yet only been met with in the Upper Chalk of Yorkshire and 
Germany. Detached spicules are, however, very abundant and widely distributed. 
They first appear in the Carboniferous strata of Ayrshire, and they also occur in 
the Lias, the Lower and Upper Greensand, the Chalk, and in the Eocene Tertiary. 
Throughout this series of rocks the spicules exhibit the same general characters as 
those of existing species of the genus. 
35. PacHAsTRELLA vetusTA, Hinde. Plate V, figs. 5, 5 a—5 c. 
1883. PacuastTreLta verusra, Hinde. Cat. Foss. Sponges, p. 209, pl. xxxvui, 
figs. 6, 6a—6 fF. 
This species includes detached spicules of the normal four-rayed type, also 
spicules in which three or five rays are present. In the four-rayed spicules, three 
of the rays are nearly in the same plane, or form the outlines of a low, three-sided 
pyramid, whilst the fourth, or vertical ray, is usually shorter than the others. In 
some spicules the vertical ray is absent, whilst in others it is prolonged beyond 
the junction with the three rays, and the spicule is then five-rayed. The rays are 
straight or slightly curved, cylindrical or gradually tapermg from the centre to 
the obtusely-pointed extremity ; occasionally the ends are digitate. As a rule the 
rays in the same spicule are unequal in length. In a small specimen the rays are 
only ‘54 mm. long by 16 mm. in thickness, whilst in a single large specimen they 
measure 6 mm. by °85 mm. 
These spicules are of somewhat rare occurrence in the Dalry decayed chert, in 
association with the detached spicules of Hyalostelia, and they are in the same 
mineral condition as these latter. A few imperfect specimens, having the same 
characters as those from Dalry, but of much smaller proportions, are present in 
calcareous shale at Clitheroe. 
Distribution.—Carboniferous : Upper part of Lower Limestone series, Cunning- 
ham Baidland, Law Quarry, Dalry, Ayrshire (J. Smith, J. Bennie). Carboniferous 
Limestone : Clitheroe, Lancashire. 
