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. Pachastbella vetusta, Hinde. Cat. Foss. Sponges, p. 209, pi. xxxviii, 



figs. 6, 6 a— 6/. 



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 tapering 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 Dairy 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 Dairy, 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, Dairy, Ayrshire (J. Smith, J. Bennie). Carboniferous 

 Limestone : Clitheroe, Lancashire. 



