The Genus Porosphcera, Steinmann. By George J. Hinde. 19 



1879. Porosj)hxra globularis, v. Zittel, Handb. der Pal., vol. i., p. 288. 



1888. „ „ H. A. Nicholson, Ann. and Mag. Nat. Hist , ser. 6, 



vol. i., p. 11. 



1889. „ „ Nicholson, Man. Pal., 3rd ed. vol. i., p. 200. 

 1889. Amorphosponyia globosa, A. Fritsch, Stud. Gebiete d. bohm. Kreiilef., 



vol. iv., p. 108, fig. 52. 

 1900. Porosphxra globularis, Eowe,Proc. Geol. Assoc, vol. xvi., pt. 6, pp. 299, 



344, 361 ; vol. xvii., pt. 1 (1901) pp. 67, 71 ; 



vol. xviii., pt, 3 (1903) pp. 37, 49. 

 1903. „ „ Steinmann, Einfiihrung in die Palaontologie, 



p. 95, fig. 125. 



Sponges simple, generally rounded, like peas or marbles, but 

 sometimes oval, loaf- or cushion-shaped, without any distinctive 

 base ; for the most part free and unattached, but in many cases 

 they grew round foreign bodies which have been incapable of 

 fossilisation, and these sponges now exhibit cylindrical hollow 

 tubes which extend partly or entirely through them (pi. I., fig. 1). 

 Generally increase of growth is uniform over the surface, but in 

 some instances fresh layers are formed so as to cover but portions 

 of the surface at once (pi. I., fig. 2).[ Small specimens are found 

 of about 1 mm. in diameter ; the larger forms range to 34 mm. in 

 diameter. 



The outer surface is completely covered with the minute 

 apertures of straight, simple canals, which radiate outwards from 

 a central point or small area, with intercalations as the sponge 

 increases in size. The apertures of the canals are rounded or 

 somewhat polygonal, from 0'16 to 0-25 mm. in diameter, and 

 they are separated from each other by the delicate mesh fibres ; 

 the interspaces being sometimes less than, and sometimes exceed- 

 ing, the width of the canal apertures. Barely, shallow open grooves 

 are faintly shown on parts of the surface of some of the larger 

 loaf-shaped forms (pi. I., figs. 9, 10). 



The four-rayed spicules which form the skeletal fibres vary 

 somewhat in size in different specimens. The pointed apical ray 

 is directed outwards ; those near the exterior project as minute 

 spines beyond the general surface of the sponge ; sometimes this 

 ray is smooth, sometimes armed with horizontal prickles. The 

 apical ray ranges from • 14 to • 35 mm. in length, and from ' 04 

 to • 1 mm. in thickness at the base. The three facial rays of the 

 spicules are shorter than the apical ray ; they curve downwards, 

 tripodal fashion, and are truncate at the ends where fused to 

 proximate spicules. They are from 0*1 to 0'2 mm. in length, and 

 from - 04 to "075 mm. in thickness. The mesh fibres formed by 

 the fusing together of the individual spicules are about O'll mm. 

 in thickness. 



The dermal layer, which is very rarely preserved, is a whitish 

 crust, uneven, and with small protuberances in places ; it consists 

 of three- and four-rayed spicules and apparently simple rod-like 



c 2 



