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



cellulis squamosis, remotis, subalternis, pustulseformihus ; ore terminali radiato." KiJlliker, 

 from an examination of one of the species {Acis guadalupensis, D. and M.), gives some 

 details of the spicules, and notes that while a prominent operculum is wanting, yet that the 

 spicules at the base of the tentacles serve the purposes of one. For a long time no additions 

 were made to the genus, one species of which, Acis nutans, Duchassaing and Michelotti, 

 seems not to be a Muriceid at all, but in 1882 Ridley [loc. cit.) described a species 

 from the Indian Ocean (Mauritius) as Acis orientalis, Ridley. A species closely related 

 to the first and the last mentioned occurs in the Challenger collection. In general habit 

 the species of this genus differ from that of the other Muriceids. The colony consists 

 of a stem, branched in one plane, on which the branches as a rule keep the one thick- 

 ness throughout. The polyps are small, with wart-like bodies usually placed at long 

 intervals on the sides of the branches and twigs. The coenenchyma is armed with 

 extraordinary large spicules, of the form of either smooth or warty sjsindles, of which 

 some six or seven, placed side by side in a longitudinal manner, surround the periphery 

 of the stem and the branches. These spicules often reach to a length of 1 to 3 mm., 

 so that they are quite perceptible to the unassisted eye. Duchassaing and Michelotti 

 allude to a layer of small scale-like deciduous spicules lying over the large spindles, but 

 no such layer exists in the East Indian species, nor is any trace of it to be found in the 

 type specimen of Acis guadalupensis in the Turin Museum, nor is it alluded to in the 

 first diagnosis of the genus. 



On the polyps the spicules are smaller, forming rings round their bodies. These 

 calcareous spicules are scale or disc-like. 



The spicules on the basal portions of the tentacles form a more or less well-developed 

 operculum. The genus in some respects seems closely related to the previously de- 

 scribed species of Echinogorgia, and also with those species of Muricella which are 

 armed with large spindles. 



Acis pustulata, n. sp. (PI. XXIV. figs. 1, la; PI. XXVII. fig. 6). 



The colony is upright, branched in one plane ; 97 mm. in height and 110 mm. in 

 breadth. The principal stem rises with a wavy outline, giving off on either side larger 

 and smaller branches, which come off at an angle of 50°, and in their turn also give off 

 large and small twigs. The larger branches give off small undivided branches, coming off' 

 laterally at right angles. The stem and branches are compressed in the plane of the 

 branches, and are throughout of about the same thickness. The terminal branches are 

 somewhat thickened at their apices. The branches frequently bend round in their course 

 and then run parallel to the stem ; in one instance there is an anastomosis of two 

 branches. The height of the principal stem is 86 mm., of the larger branches 87 mm., 

 of the terminal branches 16 mm. The principal stem has a basal diameter of 3 '5 mm., 



