334 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



that is to say, corresponding to the old genus Ophiopesa. To justify 

 placing it in the genus Pectinura, only the unique case of P. dambyi 

 (Farquhar) could be invoked, this species having the arm spines 

 very much longer than usual. But that raises the question whether 

 P. dambyi, which is only known from a single specimen from the 

 Kermadec Islands and which recalls the genus Ophiarachna by its 

 arm spines, should be retained in the genus Pectinura. I would be 

 rather disposed to place it in the genus Ophiarachna because of the 

 development of its arm spines, and these spines, some of which are 

 uppressed against the side arm plates while the others are more or 

 less divergent, strongly recall those of Ophiarachna. 



The new species differs from the species of the genus Ophiarachna 

 in two characters; there are no supplementary mouth shields and 

 the outer tentacle scale does not overlap the base of the first ventral 

 arm spine. In regard to the first difference, I may remark that the 

 occurrence of a supplementary mouth shield is never constant in 

 the species where it is usually found; thus in a specimen of O. affinis 

 from Amboina, which I have in my collection and of which I figure 

 the ventral surface (pi. 4, fig. 1), this supplementary shield is 

 lacking in one of the interradial spaces. 13 Similar variations are also 

 known in O. incrassata, as well as in the genus Pectinura. As for the 

 position of the outer tentacle scale, it occupies, as H. L. Clark has 

 remarked, the same place in the genus Ophiarachna as in the genus 

 Pectinura and in the related genera; the difference which I have 

 indicated, therefore, can not be invoked for placing our species in 

 the genus Pectinura rather than in the genus Ophiarachna. More- 

 over, it must be noticed that the overlapping of the outer tentacle 

 scale over the first ventral arm spine occurs in different degrees in 

 the genus Ophiarachna. It is only slightly marked in O. incrassata, 

 in which the two tentacle scales are very small, and very much more 

 marked in O. affinis, in which these scales are larger. 



The two differences which I have just indicated are not sufficient 

 to necessitate a generic separation, and in consequence the creation of 

 a new genus. I believe, therefore, that our species may be retained 

 in the genus Ophiarachna. 



In Ophiarachna incrassata, which is the type of the genus', there 

 is a pair of pores between the successive under arm plates, and this 

 feature occurs over a rather large portion of the length of the arms 

 with some variations in the development of these pores (pi. 4, fig. 6). 

 In the new species there are two pairs of pores only, and those of 

 the first pair, instead of being small, circular, and placed on the 



18 H. L. Clark has also published two photographs of O. affinis ('15, pi. 18, figs. 1, 2). 

 I notice that one of the mouth shields of the specimen photographed is without the 

 supplementary plate, and a second seems to me to be in the same condition. The ventral 

 pores are in two pairs, and they appear to have exactly the same characters as in the 

 specimen of which I give a photograph here ; but the outlines are not very clear. 



