FROM THE LOWER CALCAREOUS GRIT OF YORKSHIRE. 59 



Genus Rhaxella, Hinde. 



Sponges with walls of anastomosing plates or trabeeulaD, com- 

 posed entirely of aggregated masses of globate spicules. 



Khaxella perforata, sp. n. (PI. VI.) 



Sponges palmate, flabellate, or perhaps funnel-shaped ; the largest 

 specimen, which is not complete, is 140 millim. in height and SO 

 millim. in width. Outer surface smooth or faintly ribbed, perforated 

 irregularly by oval or slit-like apertures from 1 to 9 millim. in 

 diameter. The plates of the wall are from -5 to 4 millim. in 

 thickness, usually from 1 to 2 millim. They iuterosculate so as 

 to form labyrinthine interspaces of varying width. The globate 

 spicules vary from ellipsoidal to nearly spherical in form, with a 

 well-marked depression or hilum. They are mostly from -11 to 

 •15 millim. in diameter ; very small forms are only -08 millim. The 

 fibres of the globates are very minute ; their distal ends are sub- 

 circular, and average '002 millim. in diameter. 



Distribution. Corallian : Lower Calcareous Grit (Zone of Ammo- 

 nites perarmatus) : Scarborough, Yorkshire. Coral liag : Settring- 

 ton, Yorkshire {Hudleston). Detached spicules, some of which 

 probably belong to this species, also occur in the Coral liag of iS^orth 

 Grimston, Y'orkshire (Hudltston), and on nearly the same horizon 

 at Sturminster Newton, Dorsetshire, and at Hilmarton, near Colne, 

 "Wiltshire {Blake). 



Summary and Conclusions. 



In the Lower Calcareous Grit of Scarborough definite sponges of 

 somewhat irrejiular outlines, varying from funnel-shaped to sub- 

 palraate in form, occur partially weathered out, as a rule oq the 

 surface of the rock. The sponges are siliceous, their walls consist 

 of irregularly perforated plates or anastomosing trabecnla3, which 

 are entirely built up of aggregated masses of minute globate spicules 

 similar to those of the recent genera Placosponrjia and Geudia, 

 without any apparent intermixture of other spicular forms. The 

 globates show tracer of component radial fibres and surface-markings 

 precisely similar to those of analogous spicules in recent sponges. 



Sponges of this t3pe of skeletal structure, with labyrinthic walls 

 wholly of globate spicules, have not hitherto been known either 

 fossil or recent ; as a rule, spicules of this form are associated with 

 acerate, trifid or pin-shaped spicules, and in one fossil genus with 

 lithistid spicules. The recent Flacospom/ia, which has a solid 

 central axis of globate spicules, presents the nearest analogy to the 

 fossil Bliaccella. 



These sponges are very good instances of the preservation of the 

 entire or nearly entire skeleton of the organism in spite of the fact 

 that the comjjonent spicules arc not originally united together or 

 held in posidon otherwise than by the soft animal structures, which 

 necessarily perish after the death of the animal. Such instances are 

 extremely rare ; for as a rule sponges of this character fall to pieces 



