SILICEOUS SPONGES. 209 



have yielded such numerous spicules of Hexactinellid spicules of the genera Ilyalostelia 

 and Holasterella, and they closely resemble these spicules in their condition of pre- 

 servation. Seen by reflected light they have a dull white aspect ; when mounted in 

 Canada balsam and examined by transmitted light, they are nearly entirely opaque. 

 Their surfaces are generally smooth, but in places they are pitted with minute rhom- 

 bohedral cavities. In comparison with the Hexactinellid forms they are extremely 

 rare. 



Distribution. Lower Carboniferous : Law Quarry, Dairy, Ayrshire. From the 

 collection of Mr. James Bennie. 



Genus PACHASTRELLA, 0. Schmidt. 



Pachastrella vetusta, Hinde, n. sp. (Plate XXXVIII. figs. 6, 6a-/.) 



Detached four-rayed spicules ; three of the rays either nearly horizontal or forming 

 the outlines of a three-sided pyramid ; the fourth ray is vertical to the other three, 

 and forms the apex of the pyramid ; in rare instances the vertical ray is continued 

 below as well as above the pyramidal apex, and the spicule becomes five-rayed. The 

 rays are straight or slightly curved, cylindrical, or very gradually tapering to the 

 extremity, which is usually obtusely rounded, though in one instance the rays termi- 

 nate in minute digitations. As a rule, the rays in each spicule are unequal in length, 

 the vertical ray being usually the shortest. The spicules vary greatly in size ; in a 

 large form the rays reach to 2*6 mm. in length by "47 mm. in thickness, whilst the 

 rays in a small spicule are not more than -54 mm. in length by "IG in thickness. 



The close resemblance in the general characters of these spicules to those of the 

 Cretaceous species of Pachastrella, indicates that this genus dates back to the Carbo- 

 niferous period*. As a rule, the spicules are larger than in the forms from the 

 Upper Chalk. They occur under the same conditions as the forms described above, 

 and are comparatively rare. 



Distribution. Lower Carboniferous : Law Quarry, Dairy, Ayrshire. From the 

 collection of Mr. James Bennie. 



* The late Mr. Charles Moore, F.G.S., descrihed and figured under the name of Grantia antiqua (Quart. 

 Journ. Gcol. Soc. vol. sxiii. 1867, p. 538, pi. xvi. f. 33, 34) some detached triradiate calcareous spicides from 

 the Liassic Conglomerates at Brocastle, South Wales. It is very doubtful if these forms were originally 

 calcareous ; they may probably have been siliceous spicules allied to Pachastrella, the structure of which has 

 been replaced by calcite. 



2e 



