236 
THE VOYAGE OF H.M.S. CHALLENGER, 
Genus 7. Isops, Sollas. 
Oscules and pores similar, both the simple apertures of similar chones. 
Isops pachydermata, Sollas. 
Isops paeliydermata, Sollas, Prelim. Account, Sci. Proc. Roy. Dubl. Soc., vol. v. p. 198, 1886. 
Sponge. — An irregular rounded mass ; attached. Surface smooth, raised at intervals 
of about 7 mm. into small rounded bosses or tubercles, each perforated centrally by a single 
aperture, the distal mouth of a chone. Oscules and pores similar, each the simple 
mouth of a chone. Cortex thick, constituted almost entirely by the sterrastral layer ; 
beneath the epithelium a layer of spherasters. 
Spicules. — I. Megascleres. 1. Oxea, fusiform, straight or more usually curved, bluntly 
pointed or strongylate ; 1‘96 by 0'04 mm. 
2. Orthotridsne, rhabdome conical, strongylate; cladi, conical, straight, projecting 
outwards and slightly forwards. Ehabdome 1'07 by 0'0387 mm., cladi 0’27 mm. long. 
II. Microscleres. 3, Sterraster, more or less ellipsoidal, compressed, actines bearing 
from four to six small recurved spines ; 0'24by0’187 mm. in diameter. 
4. Somal spheraster, minute, a comparatively large centrum, bearing numerous short, 
cylindrical, abruptly truncate actines. Centrum 0’008 mm. in diameter, actines about 
0'003 mm. in length, total diameter 0'014 mm. 
5. Choanosomal oxy aster; no differentiated centrum, actines long, slender, conical, 
sharply pointed, few in number, varying from eight to two, those with only two actines 
being frequently microxeas ; a single actine of a triod form 0'064 mm. in length. 
6. Subcortical oxeate spheraster ; this form is intermediate between the two preceding ; 
centrum well developed, actines numerous, sharply pointed, conical, rarely faintly and 
minutely spined near the extremities, a single actine O’Ol mm. in length ; total 
diameter, 0*035 mm. 
Colour. — Yellowish-white. 
Habitat. — Station 56, off Bermuda, May 29, 1873 ; lat. 32° 8'45"N., long. 64° 59' 35" 
W.; depth, 1075 fathoms ; bottom, coral mud; bottom temperature, 38°*2. Dredged. 
Remarhs. — A single imperfect specimen of this sponge was dredged ; it has grown 
over the hard skeleton of a species of Isis, the polyps of which are still present atone end; 
a part of the polypary has extended itself over the surface of the sponge. In the characters 
of its spicules the sponge resembles Geodia megastrella, Carter, and in the structure of the 
cortex it brings to mind Cydonium hirsutus, Sollas. It is readily distinguished from 
both these species by the characters of the chones, which are those of an Isops. 
The cortex is 1*68 mm. in thickness, it consists almost entirely of the sterrastral layer. 
