552 PEOF. W. J. SOLLAS ON SOME EOSSIL SPONGES 



one ; for Oscar Schmidt, in his last published work on the Mexican 

 sponges, describes several species of Lithistids and Dictyonine 

 Hexactinellids from a depth of 100 fathoms * ; one of the latter, 

 indeed (Cystispongia superstes), dredged off Yucatan, comes from 

 the comparatively shallow water of 18 fathoms. Moreover, after a 

 preliminary examination of the ' Challenger ' Tetractinellidse, I find 

 one species of Lithistid which was obtained from a depth of 

 18-20 fathoms. If, then, Lithistids and Hexactinellids, at the 

 present day, extend into water only 18 fathoms in depth, it is no 

 longer possible to regard their fossil representatives as indicative of 

 deep-sea conditions. 



All that we know of the Inferior-Oolite rocks of England points 

 to their having been deposited in shallow water in proximity to a 

 shore-line ; and the particular beds in which the just-described 

 Hexactinellids and Lithistids occur bear every stamp of a shallow - 

 water origin : they are impure limestones and shelly breccias, some- 

 times false-bedded and sometimes oolitic in structure. The mol- 

 luscan fauna also appears not to be that of a deep sea ; and the 

 genus Litorina points even to shore-conditions. 



That which chiefly conditions the existence of Lithistid and 

 Hexactinellid sponges appears to be uniformity of temperature. 

 This, as all available evidence unites in showing, was an enduring 

 characteristic of our area during Oolitic times. 



Through the persistent summer of this period Hexactinellids and 

 Lithistids flourished, along with reef-building corals, in shallow 

 seas, secured from severe seasonal changes of temperature, just as, 

 at the present day, they inhabit the shallow water which washes 

 the shores of coral reefs in the uniform climate of the tropics. 



EXPLANATION OF PLATES XX. & XXI. 

 Plate XX. 



Figs. 1-6. Emploca ovata (gen. et sp. nov.). 



Figs. 1 & 2. The sponge: natural size. 



3 & 4. Longitudinal and transverse sections across the canals, showing 



the axial thread of calcite (c), the granular matrix (m), and the 



surrounding skeleton (s) : ( X 22*5). 

 Fig- 5. Portion of the skeletal network seen in section : ( X 45). 



6. External surface of the sponge, showing the projecting ends of the 



external rays of the skeleton : ( X 45). 



Figs. 7-9. Mastodictyum Whidbomi (gen. et sp. nor.). 



7. A part of the sponge : natural size. 



Figs. 8 & 9. The skeletal network shown in section : ( X 45). 



* On tabulating O. Schmidt's recorded occurrences of the Dictyonine Hexac- 

 tinellids, I find 22 cases of species dredged from between 100 and 200 fathoms, 

 12 from 200 to 300, 4 from 300 to 400, 2 from 400 to 500, 1 from 500 to 600, 

 2 from 600 to 700, 2 from 700 to 800, 4 from 800 to 900, 1 from 900 to 1000, 

 and 4 from 1000 to 2500 fathoms. Thus these sponges are more frequent 

 between 100 and 300 fathoms than in greater depths, and it may be added 

 that they appear to be most abundant between depths of 100 and 150 fathoms. 



