PORIFERA. III. I2 i 



This species is related to H. umbellifera Tops., but besides other characters this latter species 

 has only one form of ancorse. 



Locality: Station 89, the Denmark Strait, 64 45' L,at. N., 27 20' L,ong. W., depth 310 fathoms; 

 two specimens. 



4. H. tenuisclera n. sp. 

 PI. XI, Fig. 7. 



Incrusting ; surface smooth. Spicula: megasclera; the skeletal spicules acanthostyli with a small 

 but generally distinct head, somewhat densely spined in the whole length, o^oSj (rip mm , not divided 

 into two groups ; the dermal spicules strongyla o'/j 0-21""*; microsclera small isancora unguiferce with 

 5 8 teeth at each end, 0-016 0-020""". 



This species forms incrustations on Bryozoa (Retepora and Cellepora), on Brachiopods and on 

 tubes of Placostegus tridentahis. Its greatest extent is about 15 mm , and it is very thin, not reaching 

 0-5 mm . The colour (in spirit) is whitish. The surface is smooth or it may be finely hispid from pro- 

 jecting acanthostyli, but this latter condition is certainly due to some damage. The dermal membrane 

 is a thin and transparent film. Oscula and pores were not observed. 



The skeleton. The dermal skeleton consists of bundles of dermal spicules stretching from the 

 skeleton below to the dermal membrane, the spicules are somewhat penicillately spread outwards; 

 besides these bundles some more irregularly scattered spicules are seen; the bundles and spicules are 

 not at all densely placed, and the dermal skeleton is somewhat weakly developed. The main skeleton 

 is of typical construction, and it is rather regular, consisting of vertical acanthostyli. At the base 

 there is a very small amount of spongin, only observable with difficulty. 



Spicula: a. Megasclera. i. The skeletal spicules are acanthostyli; they are generally straight, 

 only rarely very slightly curved; the head is not large but as a rule distinct, and the shaft tapers 

 evenly from the head to the point The styli are somewhat densely spined in their whole length, 

 sometimes the spines may be a little more dispersed towards the point; the spines are medium sized 

 and reclined, on the head they are somewhat long, blunt and radiating straight out The styli vary 

 somewhat in length, but they are not divided into groups; the length is 0-083 0-19""" an< ^ t ^ le diameter 

 of the head o-on 0-024 mm . 2. The dermal spicules are straight and slender strongyla; they are of 

 the same thickness in the whole length and sometimes they are a little polytylote; the ends may 

 be slightly swollen. The length is 0-15 o-2i mm and the diameter 0-0028 0-004 mm . b- Microsclera; 

 these are isan corse anguiferse; they are small and have a slender, curved shaft and five to eight 

 narrow teeth at each end; there may be some irregularity present, the teeth sometimes being of 

 different length and breadth and often the number of the teeth is different at the two ends; there 

 are very narrow alse on the shaft, but they are difficult to observe. The curvature of the shaft is 

 generally very strong, but there is some variation on this point, especially in different individuals. 

 The length of the ancorse, which is somewhat dependent on the curvature, is 0-016 0-020 mm , and the 

 diameter of the shaft is about o-ooi mm . The ancorae are present in great numbers in the dermal 

 membrane but occur also otherwise in the tissue. 



This species is very nearly related to H. biscutella Tops., but it is distinguished by several 



The Ingolf-Expedition. VI. 3. l6 



