16 Transactions of the Society. 



layer consists of very slender rod-like spicules, regularly disposed 

 side by side or arranged concentrically round a small opening 

 (? pore). (PI. II., fig. 10.) 



Out of a total number of about 3000 specimens of Porosphcera 

 examined, I have only met with a dermal layer in eighteen indi- 

 viduals. In but two of these does it extend over the whole surface ; 

 in the others only small patches of the skeletal mesh are now 

 covered by it. Where thin, the apical spicular rays can be seen 

 to penetrate through it. It occurs in specimens of P. globularis, 

 P. nuciformis, P. pileolus and P. patelliformis, from different 

 localities and horizons, from the zone of Micrastcr cor-testudinarium 

 upwards, with the exception of the zone of M. cor-anguinum. Pro- 

 portionately, a larger number of specimens with the dermal layer 

 partially preserved are found in the zone of Bel. mucronata than in 

 the lower horizons of the English Chalk. 



In its structure and general characters, the dermal layer of 

 Porosplmra is very similar to that of the Tertiary Plectroninia and 

 the recent Pctrostroma from the Japanese Sea. In both of these 



EXPLANATION OF PLATE II. 



Fig. 1. Porosph.xra globularis, Phill., sp. A small specimen, preserved in flint, 



showing the spicular structure of the exterior. x 50 diam. Upper 



Cbalk ; near Sidcup, Kent. Collection of Mr. H. Muller. 

 ., 2. Ditto ; portion of the outer surface, showing the arrangement of the skeletal 



spicules bounding the apertures of the radial canals. X 40 diam. Zone 



of Belemnitella mucronata ; Ballard Cliff, Dorset coast. Collection of 



Dr. Rowe. 

 „ 3. Ditto; three-rayed spicules of the dermal layer, x 100 diam. Zone of 



Aclinocamax quadratus; Scralchell's Bay, Isle of Wight. Collection of 



G. J. Hinde. 

 „ 4. Porosphxra nuciformis, \. Hag., sp. Portion of the surface, showing the 



skeletal spicules and the radial canal apertures, x 40 diam. Zone of 



A. quadratus ; Cliff, east of Brighton. 

 ., 5. rorosphxra pileolus. A four-rayed mesh spicule, the apical ray armed with 



lateral prickles. From a microscopic section near the margin of the 



specimen, x 200 diam. Zone of Micrader cor-anguinum ; South Croydon. 



Collection of G. J. Hinde. 

 ,, (I. P. globularis. A portion of the skeletal mesh, showing its structure of fuur- 



rayed spicules, the basal rays of which are now fused together, x 100 diam. 



Zone of M. cor-anguinum ; South Croydon. Collection of G. J. Hinde. 

 „ 7. Ditto; a fragment of the dermal layer, showing three- and four-rayed 



spicules irregularly intermingled, x 50 diam. Zone of B. mucronata ; 



Ballard Cliff, Dorset coast. Collection of Dr. Rowe. 

 ., IS. Ditto; a small specimen preserved in chalk, showing blunted apical rays of 



spicules projecting from the surface, x 40 diam. Upper Cbalk, Graves- 

 end, Kent. Collection of Mr. T. H. Powell. 

 „ 9. Ditto; the skeletal mesh near the margin of a specimen preserved in flint, 



showing the curved facial and the projecting apical rays of four-rayed 



spicules, x 40 diam. Upper Chalk; Chatham. Jeimyn Street Museum. 

 „" 10. Ditto; portion of the outer surface of the dermal layer, showing rod-like 



spicules arranged concentrically round a central pore (?). x 50 diam. 



Zone of Belemnitella mucronata; Ballard CI iff', Dorset coast. Collection 



of Dr. Bowe. 



