The Genus Porosphcera, Steinmann. By George J. Hindc. 23 



Sponges simple, free, thimble- or inverted cup-shaped, some- 

 times hemispherical, with a deeply concave, cup-shaped base. 

 Walls thick. The outer surface is even, without grooves or ridges, 

 and covered with the apertures of the minute radial canals. The 

 concave base shows concentric rings or bands of growth, and a small 

 umbo or boss at the bottom. The specimens range from 2 to 18 mm. 

 in diameter. 



The skeletal fibres resemble those of P. globularis. The canals 

 radiate upwards from the basal layer. The concave base has a 

 layer of minute spicular rays, regularly arranged like thatch on a 

 roof, and outside of this there appear to have been elongate, slender, 

 rod-like spicules. 



In one specimen there is a small fragment remaining of a 

 spicular dermal crust of a similar character to that in P. globularis. 



This species is characterised by its form, thick wall, and deeply 

 concave base. 



The various specific names by which this species has been 

 known, are taken from Lamarck's Animaux sans Vcrtebrcs, torn. ii. 

 (1816) pp. 190-197. They were applied originally to Foraminifera 

 or Polyzoa, and therefore have no proper connection with this 

 sponge. It seems to me desirable, however, that quite inde- 

 pendently of Lamarck's use of the term " pileolus" it may be 

 suitably retained for this species of Porosphcera. 



Distribution. — P. pileolus is first known from the Holaster planus 

 zone at Dover, and from this upwards it occurs in the higher beds 

 of the Chalk to the top of the Actinocamax quadratics zone at 

 Croydon, Margate, Thanet coast, near Newhaven, near Brighton, 

 Isle of Wight, Dorset coast, as well as at Flamborough and 

 Sewerby, Yorkshire. 



Porosphcera patelliformis, sp. n. (pi. I., figs. 22-2 6ft). 



1822. Lunulites (?) Mantell, Geology of Sussex, p. 180, pi. xvi., figs. 22-24. 

 1835. Orbitolites lenticulata, Mantell (nou Lam.), Trans. Geol. Soc, ser. 2, 

 vol. iii,. p. 204. 



Sponges limpet-shaped, with peaked or rounded summits, base 

 rounded or oval in outline, usually deeply concave, but occasionally 

 flattened, and in some young specimens even slightly convex. 

 Wall relatively thin. The specimens range from 2 to 23 mm. in 

 diameter. 



The skeletal fibres are of a somewhat coarse character, and the 

 radial canals are short ; their apertures range to • 3 mm. in width. 

 The concave base shows concentric bands of growth, and its 

 spicular structure is similar to that of P. 'pileolus. Usually there is 

 no boss at the bottom of the base. A spicular dermal crust covers 

 in part the surface of one specimen (pi. I., fig. 25). 



This species is distinguished from P. pileolus by its limpet-like 



