OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 335 



distal border of the first under arm plate, are on the sides of the 

 latter; they are much elongated and form a small and very evident 

 groove. This arrangement is therefore very different from that 

 which exists in O. incrassata, but it recalls very strongly that in 

 0, affinis. In this species the ventral pores are very much less 

 numerous than in O. incrassata, and usually number two pairs only ; 

 those of the first pair are situated on the sides of the first under arm 

 plate and not on its distal border, and they are more or less elongated 

 as in the new species. In a specimen of 0. affinis which is in my 

 collection and which comes from Amboina (pi. 4, fig. 1), there are, 

 as a rule, two pairs of ventral pores; the pores of the second pair, 

 instead of being elongated like those of the first pair, are circular 

 and situated on the distal border of the under arm plate; they are 

 also only slightly developed, and on one of the arms they are almost 

 invisible ; on the other hand, on another arm I find traces of a third 

 pair. The Geneva Museum has two specimens of O. affinis belong- 

 ing to Loriol's collection, and also from Amboina. I have examined 

 the arrangement of their ventral pores, and I find that they show 

 interesting variations. In the larger specimen, in which the disk 

 has a diameter of 15.5 mm., there exists first between the first and 

 the second under arm plates a pair of pores identical with those 

 which I have noticed above; these pores, always situated on the 

 sides of the first under arm plate, are elongated and narrow, but 

 they are extremely well marked. Furthermore, there occurs between 

 the following under arm plates a series of small rounded pores ar- 

 ranged as in O. incrassata — that is, placed on the distal border of the 

 plates ; I am able to distinguish four successive pairs of these rounded 

 pores. In the second specimen, which is smaller (the diameter of 

 the disk is 12 mm. ) , the pores of the first pair are extremely narrow, 

 and I can not distinguish beyond them but a single pair of rounded 

 pores of very small size; but on two arms I recognize the traces of 

 a third pair. 



This shows that the ventral pores of 0. affinis may vary in their 

 development as well as in their number; there are always, however, 

 at least two pairs of them. This is also the number which I find 

 in two other species of the genus 0. mauritiensis and 0. roMUardi, 

 described by Loriol. But in describing these species the learned 

 Swiss naturalist did not mention the ventral pores, and his figures 

 also do not show the least trace of them. Having had an oppor- 

 tunity of studying Loriol's types at the Geneva Museum I am able 

 to make good this slight deficiency. Ophiarachna mauritiensis is 

 represented by two specimens in which the ventral pores are ar- 

 ranged exactly as in my specimen of O. affinis, and they quite agree 

 with the photograph which I give here of this latter species (pi. 4, 

 fig. 1). In O. robillardi, which is only represented by one rather 



