192 



PORIFERA. II. 



almost quite, symmetrical with uniform teeth all round, and the shaft is frequently attached inside the 

 circle of teeth, so that all the teeth are free, and then we have really a birotula with curved shaft; 

 one or two teeth, however, are also often here prolonged and more or less united with the dorsal side 

 of the shaft. Their length is 0-030— o - 040 mm , and the thickness of the shaft is 0-0021 — o-oo35 mm . A few 

 developmental forms were seen; the beginning teeth are seen in these, while the shaft is still rather 

 thin. 2. Birotula;; the}' have nine to twelve teeth at either end, the length is o - oi8 — o - 032 mm , the 

 latter length, however, is only rarely found, the thickness of the shaft is o - oon— o-ooi4 mm , and the 

 breadth of the circle of teeth is 0-0057 — O'ooj" 1 " 1 . The largest birotuke may have up to thirteen teeth, 

 also thereby showing that the}- form a transition to the ancorse. A few developmental forms of birotulse 

 were seen; like those of the ancorse they consist of a shaft with a knob at either end. and the knobs 

 showed slight ribs as a beginning of the teeth. The microsclera occur throughout the sponge, the 

 ancorse are frequent in the dermal membrane; birotulse are most numerous. 



Locality: Rathboue Island off the Liverpool-Coast on the eastern coast of Greenland, ca. 70" 40' 

 Lat. N., depth 94 fathoms (the Amdrup-Expedition). 



7. I. polydentata u. sp. 



PI. VI, Fig. 7. PI. XVIII, Fig. 7 a-e. 



Erect, wore or less irregularly leaf-shaped, or the leaf-shape effaced, passing into a more massive 

 forjn. The surface somezvhat grooved, not shaggy. The dermal membrane not especially thin, on the 

 pore side supported by recumbent bundles or short fibres of dermal spicules, and also provided with 

 scattered, horizontal spicules ; on the opposite side only horizo/ital, scattered spicules are found, Oscula 

 scattered on both surfaces. The skeleton an irregular, mostly polyspicular reticulation, with loose fibres 

 running in the longitudinal direction. Spicula : Afegasclera : the skeletal spicules styli 0-42 — o-^y m '", the 

 dermal spicules tylota 0-25 — 0-34"""; microsclera two forms of birotula, large ones with twelve to twenty 

 teeth o-02o — 0-028""", small ones with nine to fourteen teeth 0-0128 — o-oi8'" m . 



The largest and finest developed specimen of this species is formed like an erect, rather thick 

 leaf, another, smaller specimen is of a similar form, while a third, also smaller specimen is thicker and 

 more irregular, still, however, with a tendency towards the leaf-shape. According to this, the typical 

 form of the species must be supposed to be as an erect, more or less irregular leaf. The largest speci- 

 men has a height of 70 mm , a similar breadth, and a thickness of ca. 20 min . The colour (in spirit) is 

 grayish brown. The consistency is somewhat elastic, but the sponge is rather brittle. The surface 

 is somewhat grooved, but otherwise smooth; the grooves arise in the common way by the dermal 

 membrane being sunk over the subdermal cavities, which shine through as dark areas. This con- 

 struction is distinctly seen in one specimen, but not in the two others, which are more compact, pre- 

 sumably highly contracted. The dermal membrane is not especially thin, and in places where the 

 sponge is contracted it becomes still a little thicker; it is supported by dermal spicules, partly in 

 bundles, partly scattered. The pores are seen in sieve-like gatherings in the skin above the subdermal 

 cavities, they are really found over the whole surface, but are not conspicuous where the membrane 

 passes the parts of tissue separating the subdermal cavities; they are only found on one surface of 



