66 THE VOYAGE OF H.M.S. CHALLENGER. 



quadrangular lobes and deep incisions. Surface reticular, pierced by numerous holes, 

 very soft, without concentric zones, but with more or less distinct ribs or branched 

 veins. Skeleton composed mainly of Eadiolarian shells and siliceous sponge spicules, 

 intermingled in the ribs with numerous Globig evince ; spongin-fibrilke thin and of 

 nearly equal breadth, loosely interwoven. 



Stannophyllum pertusum differs externally very strikingly from the other species of 

 the genus by the numerous lobes of the distal margin and the holes which pierce the 

 reniform leaf and produce its reticular appearance ; these characters, however, are fouud 

 less prominently in the closely-allied Stannophyllum venosum, with which it is 

 connected hy intermediate forms. The branched ribs of the latter species, too, are 

 usually more or less indicated, but rarely so prominent. The shape of the two species 

 is also similar, produced by the same composition of the skeleton. This contains a loose 

 framework of interwoven spongin-fibrillse of various sizes, and imbedded in its meshes 

 a variable quantity and quality of xenophya. 



Xenophya. — The aggregation of foreign bodies which compose the pseudo-skeleton 

 is a variable mixture of Eadiolarian remains and of Globigerina ooze, the latter usually 

 predominating in the proximal portion of the leaf, its ribs and the pedicle, the former 

 in the distal portion and in the thin lamella? between the ribs. The numerous specimens 

 in the Challenger collection vary a great deal in this respect ; when the siliceous shells 

 of Radiolaria are predominant, the structure of the leaf approaches that of Stanno- 

 phyllum radiolarium ; when the calcareous shells of the Foraminifera are abundant, 

 it is more like that of Stannophyllum globigerinum. The spongin-fibrillse are also very 

 variable, thinner and finer in the former, thicker and coarser in the latter. The leaf of 

 the latter is far more flabby, soft, and inelastic than that of the former. In some 

 specimens a great quantity of siliceous spicules of sponges (mainly Hexactinellidse) is 

 embedded in the clear maltha, and these specimens are particularly flabby and 

 easily torn. 



External Form. — The general form of the leaf in Stannophyllum pertusum is 

 kidney-shaped ; its diameter is between 80 and 120 mm., but some larger specimens seem 

 to reach 200 mm. or more, and approach near to Stannophyllum venosum. The semi- 

 circular or crescentic distal margin is always lobate, with a great number (forty to 

 sixty or more) of radial incisions, by which the rounded quadrangular lobes are 

 separated. The irregular roundish holes which pierce the flat leaf are evidently 

 produced by the growing together of formerly separated marginal lobes. Their size 

 and number is very variable. Probably the approachment of divergent branches of the 

 symbiotic reticular Hydrocaulus (Spongoxenia) is the first cause of this formation. 

 The proximal margin of the leaf is integral, broadly triangular, and tapers into a 

 slender triangular pedicle. This has a length of 30 to 50 mm., a breadth of 4 to 8 mm., 

 and is attached by a basal plate at the bottom of the sea. 



