The Genus Porosphcera, Steinniann. By George J. Ilinde. 7 



In the succeeding zones of Micraster cor-tcstudinarium and 

 M. cor-anguinum, we find P. glohularis more numerous and of 

 larger size ; in the latter zone, the ribbed, pear-shaped P. nuciformis 

 has been first noted. P. pileolus and P. patelliformis are more 

 common than in the Middle Chalk ; but they form only a small 

 percentage in comparison with the numbers of P. glohularis. 



It will be noticed that the forms from South Croydon, which I 

 have placed in the zone of Micraster cor-anguinum, differ consider- 

 ably in their maximum size from those of the same zone on the 

 Thanet coast, and. approach more nearly in this respect to those 

 obtained by Dr. Eowe from the Marsupites zone of Margate. This 

 fact has led me to question whether the Chalk at South Croydon 

 has been correctly referred to the M. cor-anguinum zone ; but I have 

 hitherto failed to find in it any remains of Marsupites. I have, 

 however, found a single specimen of faster pillula, Lam., associated 

 in the same area with the large specimens of P. glohularis ; and as 

 this echinoderm, according to Dr. Eowe, also occurs, though rarely, 

 in the Marsupites zone of Thanet, there is some probability that the 

 Chalk at South Croydon may be correlated to this zone. 



From the zone of Marsupites, at Margate more particularly, and 

 also at Flamborough, Porospthcvra is very common, and Dr. Eowe 

 has obtained a large series of the different species. The greater 

 number were found in the lower part of this zone, the Uintacrinus 

 Band of Dr. Eowe. At this horizon, in Kent, the specimens 

 generally are notably larger than those from the lower zones of the 

 Chalk, some of P. glohularis reaching a maximum diameter of 

 34 mm. ; the larger forms also are in part loaf- or cushion-shaped, 

 a feature still more strongly pronounced in specimens from the 

 higher zone of Bclemnitella mucronata. 



At various places on the Sussex coast, and at Sewerby on the 

 Yorkshire coast, Porosphcera is very abundant in the next higher 

 zone of Actinocamax quadratics. P. nuciformis is numerous in the 

 Sussex cliffs. The specimens are generally smaller than those 

 from the Marsupites zone, and approximate in size to those from 

 the Micraster cor-anguinum zone on the Thanet coast. 



In the zone of Bclemnitella mucronata, the highest in the English 

 Chalk, an interesting series of forms was collected by Dr. Eowe at 

 Ballard's Cliff and at Studland Bay, Dorset, and also from the 

 Norfolk coast. The Dorset specimens are characterised by their 

 fairly large size, the abundance of loaf- and cushion-shaped forms 

 of P. glohularis, and the relatively large proportion of P. nuciformis. 



Summing up the zonal distribution of Porosphcera in the Chalk : 

 the genus is first noted in the Lower or Grey Chalk, of the zone of 

 Holastcr suhglohosus, by a few specimens of the rare P. Woochvardi ; 

 the principal species, P. glohularis, appears for the first time in 

 the zone of B. Cuvicri, the lowest horizon of the Middle Chalk, 

 accompanied by P. arrecta. In the zone of T. gracilis, we first meet 



