49 



peduncle. In a few probably younger — specimens the peduncle is nearly cylindrical or 



c< >mj iressed cylindrical. 



The colonr of the animals is sometimes red-brown (e. g. in specimens from the Bay of 

 Bima) sometimes dirty brown (specimens from Kwadang Bay, Pajunga Island, for example). 

 The svvollen part of the peduncle is as a rule darker coloured than the inferior part, and darker 

 also than the valves. In some of the specimens the valves have slightly darker maro-ins, the 

 clark colour havingf however a blackish-brown, not a blueish tino-e. 



The spines or horns of chitin which thickly clothe the superior part of the peduncle are 

 "nearly cylindrical, irregularly curled, and nodose or slightly enlarged at intervals: the apex 

 smooth and pointed; the exterior surface longitudinally and finely ribbed" — exactly as Darwin 

 says they are in I. Cumingi. But whereas he says, that those of the latter species are "ringecl 

 with pale and dark blueish brown, which on pressure becomes slightly opalescent with pale 

 blue and hery red", those of /. sibogae never show a tracé of blue and are always yellow- or 

 brownish-yellow coloured. 



The shape of the valves corresponds in general with the description given by Darwin 

 of the valves of /. Cumingi. The tips of the valves, of the terga especially, are often broken 

 off and their exact shape is difficult to make out in consequence. The scuta have the outer surface 

 much more plainly marked by zones of growth than the terga. The valves are longitudinally 

 ribbed, the delicate ribs seem to correspond to those of the horns on the surface of the peduncle 

 in clistance from one another as well as in delicacy. 



Fig. 21, A and B of PI. IV show the internal surfaces of the valves after being cleaned 

 with caustic potash. When not separated the scutum (A) at the outer side overlaps a portion 

 of the tergum B, so that the scutal margin of the tergum (from c to d in the figure) reaches 

 as far as the pointed line to the left side of A. What Darwin calls the growing or corium- 

 covered surfaces of the valves, the parts covered with epithelium, are easily to be clistinguished 

 from the free horn-like portions. In the scutum this part is triangular but it is not of the same 

 length as the horn-like portion, as Darwin says it is in the scutum of I. Cumingi, but considerably 

 shorter. In the tergum the growing part is of an irregular diamond shape. The chitinous structure 

 of both valves is very characteristic : the figures 21 A and B try to give an idea of it. The crest 

 running from the summit of the triangular portion to the tip of the scutum, with the short lateral 

 branches directed towards the occludent margin of the valve, seems slightly more developed in 

 I. sibogae than in I. Cumingi. 



The shape of the body and the structure of the limbs in general is the same as with 

 I. Cumingi. As in that species the chitinous covering of the prosoma bears very delicate, 

 pointed hairs. I wish to point out the following, perhaps not very important differences for the 

 parts of the mouth and the other limbs. 



Mouth. The lab rum has from 3 — 7 small nob-like teeth: in younger specimens 3 or 

 4 onlv, in one of the older ones 6, in another 7. Between these nobs short and extremely 

 delicate hairs are planted. Darwin says that there are 110 teeth on the crest of the labrum in 

 I. Cumingi and that the same part in I. quadrivalvis is "a little roughened with minute points". 

 Now the teeth of the labrum of /. sibogae are hardly more than minute points ; I may add 



49 



SIKOGA-EXPEDITIE XXXI tl. 7 



