834 W. J. SOLLAS ON THE STRUCT PEE AKD 



EXPLANATION OF THE PLATES. 

 Plate XXV. 



Fig. 1. Specimen of S. pyriformis, Sow., v. S. ficus, D'Orb., v. 8. conica, 

 Court., from the Haldon Greensand, lateral view. 1 a. Its apex, 

 seen from above, showing the opening of its cloacal tube, and its 

 exposed excurrent canals. 1 b. A longitudinal section of the same 

 specimen, showing (a) its cloacal tube, (e) excurrent canals, and (i) 

 incurrent canals. (All natural size.) 



Fig. 2. Specimen of S. costata, Lamx., from the Wiltshire Greensand. la. Longi- 

 tudinal section of the same specimen. (Nat. size.) 



Fig. 3. A young form of S. piriformis, Sow. 3 a. Its apical end seen from 

 above. (Nat. size.) 



Fig. 4. Specimen of S. pyriformis, Sow., var., S. cylindrica, Court. ; lateral 

 view. 4 a. Its summit as seen from above, showing cloacal aperture 

 and exposed excurrent canals. (Nat. size.) 



Fig. 5. Part of a band of skeletal network of S. Websteri, Sow., from the longi- 

 tudinal section fig. la. ( X60.) 



Fig. 6. Specimen of 8. pyriformis, var. Fittoni, Mich., lateral view. (Nat. 

 size.) 



Fig. 7. Specimen of S. Websteri, Sow., from the Gault of Folkestone. Lateral 

 view. 7 a. Its longitudinal section. 7 b. A transverse section of the 

 same species, but from another specimen. (Nat. size.) 



Fig. 8. Specimen of 8. pyriformis, Sow., with divided summit. (Nat. size.) 



Plate XXVI. 



Fig. 1. Part of a band of skeletal network lying between two of the excurrent 

 canals of the specimen of S. Websteri, shown in pi. i. fig. 7, as seen in 

 its transverse section, fig. 7 b. a. Section of the shaft of one of its 

 component quadriradiate spicules, b. Sections of shafts, c. Three 

 arms of a quadriradiate spicule, separated from the shaft by the plane 

 of the section. ( X 60. ) 



Fig. 2. Single skeleton-spicule from the recent Lithistid sponge Discodermia 

 poly discus, Bocage. ( X 60.) 



Fig. 3. Union of the two spicular rays by the interlocking of their tubercular 

 extremities, taken from the skeleton of D. poly discus. (x60.) 



Fig. 4. A small part of the skeletal network of S. pyriformis, Sow., as exhibited 

 on its surface by reflected light. (x60.) 



Figs. 5 and ha. " Beam " spicules from Siphonia Websteri, Sow. ( X60.) 



Figs. 6 and 6 a. "Beam" spicules from the recent Discodermia poly discus, 

 Bocage. (x60.) 



Fig. 7- Skeletal network of the stem of the Siphonia, as exposed on the surface 

 of the specimen shown in PI. XXV. fig. 3. (X60.) 



Fig. 8. Specimen of the recent Lithistid sponge Discodermia po 7 ydiscus, Bocage, 

 preserved in the British Museum, o. The oscules opening into the 

 interior of the cup ; internal view. 8 a. External aspect of the same 

 specimen. (Nat. size.) 

 [Figs. 8 and 8 a are from sketches furnished me by Mr. Carter.] 



Fig. 9. One of the " dermal " spicules of the preceding, seen in place, exhi- 

 biting the characteristic central triradiate canal. 9 a. Profile view of 

 the preceding. (X60.) 



Fig. 10. Dermal spicule from the spicule-bed of the Haldon Greensand. (X7o.) 

 (After Carter, " Fossil Sponge-spicules of the Greensand compared 

 with those of existing Species," Ann. & Mag. Nat. Hist. ser. 4, vol. vii. 

 pi. ^ii. fig. 5). 



Fig. 11. Dermal spicule from the network of Siphonia costata, Lamx. ( x60.) 



Discussion. 



Mr. Charleswoeth remarked that, if the consolidation of genera 

 proposed by Mr. Sollas were satisfactorily established, it must be re- 



