177 



portions of articulated skeleton persist, the monaxon spicules 

 are only few in number, but the latter become abundant where 

 the desmas are few in number or absent. 



The specimen (Fig. 7) encrusts Pachastrella isorrhopa Kpk, 

 and in its complete state (for part of it has been cut off and 

 retained by Dr. Gilchrist) it probably formed a circular patch 

 about 5 cm. in diameter, '6 cm. thick in the centre, and sloping 

 down to a thin rounded margin. 



A second specimen (Fig. 6) of what appeals to me to belong 

 to the same species, and which I name var. tubnUitns, presents 

 in its mode of growth certain remarkable features which seem 

 to be due to adaptation. 



In a deep fissure in a specimen of PacJiastrclla isoirhopa 

 there were two white tubes, 2.3 cm. in length and 2 mm. in 

 diameter. On cutting into the PachastrcUa the tubes were seen 

 to emerge from an irregularly shaped nodule about 25 mm. in 

 diameter embedded in the sponge, and only distinguished from 

 the rest of the Pachasiiclla by a slight difference in shade. 



The oscular tubes are composed of two layers, an inner 

 formed of fan-shaped bundles of oxeas, arranged spirally and 

 obliquely upwards, and an outer layer formed of a felt work of 

 oxeas. 



The nodule is formed of bundles of oxeas and styles and of 

 patches of monocrepid desmas of nearly the same character 

 of those of the above described type specimen ; the oxeas 

 and styles were usually slightly larger, viz., about 9 00 /^, 

 and the desmas thicker and with larger articular surfaces. 



Although the body of the Lithistid is completely surrounded 

 and, to all appearances, incorporated, yet under the microscope 

 the thin line of the dermal membrane of the Pachastrella can 

 be made out. 



Carter points out (Ann. Mag. N. H. 1876 (4), xviii., p. 410) 

 that it is a characteristic of Pachastrella to incorporate any 

 objects with which it comes in contact. In the case of the 

 second specimen of M. pluniosus, the oscular tubes appear to 

 have arisen in response to the needs of the sponge, so nearly 

 engulfed by the Pachastrella on which it grew. 



The inclusion of Monanthns in the family Desnianthidce [see 

 M^m, Soc. Zool France, 1898, xi., p. 231) renders necessary a 

 slight en'argement of Topsent's definition, viz. : to "megascleres 

 monactinaux," to add "ou diactinaux." 



Localities : Both the typical form and var. tubulatus, en- 

 crusting or invested by Pachastrella isorrhopa, from the Natal 

 Coast (Cone Point bearing N.W. i W., distant 4 miles) depth, 

 34 fathoms ; bottom, broken shells. The typical form also 

 from E. London Coast (lat. 33° 6' 30" S., long. 28° 11' E.) 

 ^epth, 85 fathoms, encrusting Placospongia lahyrinthica. 



