PHARETEOSPONGIA.— PACHYTILODIA. 203 



Mr. Carter (though not by Prof. Sollas) are acknowledged to be calcareous sponges, 

 whilst their condition is altogetlier different from that of the undisputed siliceous 

 sponges in the same beds. As, however, some of the siliceous sponges from the 

 Cambridge Green Sand are probably derived from other deposits, the dissimilarity 

 existing between them and the specimens of Pharetrospongia may be alleged to be 

 owing to the different conditions to which they have respectively been exposed ; but 

 the argument is quite applicable to the examples from the Upper Chalk ; and if 

 there are important differences in the character and mineral structure of examples of 

 Pharetrospongia, and of undisputed siliceous sponges from the same beds and the 

 same localities, they may reasonably be supposed to be due to differences in their 

 original condition. Thus, for example, specimens of the calcareous sponges Elas- 

 mostoma scitulum and Pharetrospongm Strahani, from the Upper Chalk at Bromley 

 in Kent, alike retain the perfect form and calcareous structure of their component 

 fibres ; but in the undoubtedly siliceous sponges in the same beds and locality, such 

 as Ventriculites or Scyphia, the fibres or mesh, instead of being perfectly smooth and 

 calcareous, as in the first-named forms, have been either completely dissolved, leavin» 

 empty moulds, or else replaced by powdery iron peroxide. It is impossible to 

 explain these notable differences without supposing that the mineral structure of 

 these sponges was originally different, and it seems therefore reasonable to conclude 

 that the forms with the calcareous fibres retain their original constitution. Mr. Carter 

 readily acknowledges that Elasmostoma scitulum is a calcisponge ; and Pharetro- 

 spongia Strahani so closely resembles it in general structure that, notwithstanding 

 the different form of its component spicules, there can hardly be a doubt of its 

 calcareous origin. 



Distribution. Upper Green Sand : Warminster, near Cambridge. Upper (Ihalk : 

 Charlton, Bromley, Broadstairs, Kent ; Ditchhampton, near Wilton, Norwich ; 

 Pertwood, near Warminster. 



Genus PACHYTILODIA, Zittcl, 1878. 



Pachttilodia infundibulifoemis, Goldf. sp. 



1826-33. Scyphia infundibuliformis, Goldf. Petref. 1 Th. p. 12, t. 5. f. 2. 

 1874. Cupulospongia infundibuliformis, Gein. Pal. Bd. 20, p. 29, t. 4. f. 4, 5. 

 1878. Scyphia infundibuliformis, Quenst. Petref. Bd. 5, p. 347, t. 132. f. 1-3. 

 1878. Pachyiilodia infundibuliformis, Zitt. Studien, III Ab. p. 46. 

 1883. Pachyiilodia infundibuliformis, Dun. Pal. Bd. 29, p. 42, t. 40. f. 1, 2. 



Sponges cup- or vase-shaped, supported on thickened nodose stems, occasionally 

 forming convolute extensions. An imperfect specimen measures 140 mm. in heio-ht 

 by 170 in width. The walls are from 8 to 25 mm. in thickness; the interspaces 

 between the fibres are relatively large. The fibres are from -4 to -9 mm. in thickness ; 



2d 2 



