REPORT ON THE CALCAREA. 71 



lustre. The convex surface bears low volcano-like oscula, disposed at approximately equal 

 distances one from another; their diameter does not exceed - 4 mm., usually being still 

 less. The concave surface may be compared fco a sieve, its pores, inconspicuous to the 

 naked eye, are found under the microscope to be round and disposed close together; their 

 average diameter is - 06 mm. (PI. IX. fig. 3). The walls of the calyx, 3 mm. to 4 mm. 

 thick near the centre, grow gradually thinner to its free blade-like margin. 



The species is represented in the collection by two complete specimens, one young, 7 

 mm. broad, 4 - 5 mm. high, with but few oscula ; the other of considerably larger size (the 

 longer diameter of the calyx 35 mm., the shorter 22 mm., the height 20 mm.), and by 

 a fragment belonging apparently to a specimen of still larger dimensions. The larger 

 complete specimen is represented on PI. II. fig. 7. 



The internal organisation (PL IX. fig. 1) does not differ from that of Leuconidse. 



Skeleton. — The skeleton of the sieve-like surface consists of sagittal triradiate and 

 minute acerate spicules ; that of the parenchyma of large regular, often sagittal triradiate, 

 and of minute acerate spicules ; that of the convex oscular surface of large acerate and 

 subdermal triradiate ; that of the oscula themselves of an exterior layer of large acerate, 

 of a middle layer of sagittal triradiate, of an inner layer of quadriradiate, and of minute 

 acerate spicules, supporting the ring-like border at the external opening of the osculum 

 (PL IX. fig. 2). The minute acerate spicules are in all parts of the body of the Sponge of 

 ■ he same outline. 



Minute acerate spicules. — The most typical variations are given on PI. X. fig. 10. Usually - 05 

 mm. long, with a diameter of 0'0025 mm. 



Triradiate spicules of the sieve-like surface. — Sagittal ; all rays lying in the same plane, of the 

 same diameter, tapering from the base to approximately sharp points ; lateral rays curved 

 forwards, slightly undulating, each forming with basal ray an angle varying from 115° to 

 120°, reaching 0'75 mm. in length (usually not longer than - 5 mm., often still shorter) 

 the proportion between the length and the thickness being 15 : 1; basal ray straight, length 

 inconstant, either rather exceeding that of lateral rays or equal to it, or even shorter. 



Triradiati spicules of the parenchyma. — Regular, with pronounced inclination to sagittal 

 differentiation by the shortening of basal ray ; all rays of the same diameter, the propor- 

 tion between their length ami thickness varying (in lateral rays) from 10 : 1 to 12:1; lateral 

 rays smooth, tapering from the base to sharp points ; basal ray, if not shortened, also 

 sharp-pointed, if shortened, often truncated, in both cases, however, of conical form ; size 

 extremely inconstant, the length varying (in lateral rays) from 0*15 mm. to 1'8 mm. In 

 two cases I found these triradiate spicules showing a rudimentary fourth apical ray. 



Subdermal tri/radiati spieuli s of the convex surface. — Sagittal ; all rays lying in the same plane ; 

 basal ray straight, tapering from the base to a sharp point, J-| as thick as lateral rays 

 and either longer than these latter (not more than twice, however), or of the same length, 

 or even shorter, forming with each of them an angle varying from 110° to 115°; lateral 

 rays either straight, or slightly curved, average length 0'6 mm., the proportion betwet i 



