24 Transactions of the Society. 



form, thin walls, slight development of the radial canals, and some- 

 what coarser skeletal fibres. 



Distribution. — It occurs, rarely, in the zones of Tcreuratulina 

 gracilis and Holastcr planus at Dover, and in each succeeding 

 higher zone of the Chalk. It is most abundant in the M. cor- 

 testudinarium zone near Newhaven, in the Uintacrinus band of the 

 Thanet coast, and at Flamborougli. 



Poi'osjrfio'ra arrccta, sp. n. (pi. I., figs. 27-28a). 



Sponges small, simple, conical pillar-shaped, the base concave 

 with thin margins ; in some specimens it retains traces of spicules, 

 whilst in others it is rugose, as if it had been attached to an un- 

 even surface originally. The sponges are about 8 mm. in height, 

 and the diameter of the base from 3 to 7 mm. 



The walls are thin and the canals are scarcely noticeable; 

 whilst the skeletal fibres are of the usual character. 



This form is rare ; it first appears in the zone of B. Cuvieri at 

 Branscombe and Barry Cliff on the South Devon coast ; it occurs 

 also in the zone of M. cor-anguinum at Flamborougli, in the 

 Marsupitcs zone at Margate, and in the A. quadratics zone near 

 Newhaven. 



Porosphccra. Irregular forms. 



There are a few specimens in the collections examined which 

 differ from any of the species described above, but do not present 

 any features sufficiently well marked to justify placing them as 

 distinct forms. Some are merely thin crusts either overlapping 

 one another, or attached to other bodies, others are spindle-shaped 

 and free, whilst yet others appear to be distorted or monstrous 

 growths of P. globularis and P. patelliformis. 



XII. Summary. 



The descriptions of the structure of Porosphccra given in this 

 paper are based chiefly on extensive collections from the various 

 zones of the English Chalk made by Dr. A. W. Bowe and by the 

 author. The fossils have long been well known, but owing to 

 their state of preservation, it has been difficult to ascertain their 

 minute structure, and hence very divergent opinions have been 

 held respecting their nature and systematic position ; latterly, 

 however, the view that they were Hydrozoa, structurally allied to 

 Millcpora and Parkeria, has been generally accepted. It is now 

 definitely shown that the calcareous anastomosing fibres of their 



