163 



than three times, the other arms about twice the body length. They have 

 a slight widening at the point. Although the skeleton is dissolved, it can 

 be definitely ascertained that the posterolateral rods are thorny along their 

 outer sides. 



There seems to be reason to think that this species is related to 0. arcifer, 

 but, of course, this cannot be definitely ascertained until its skeletal struc- 

 ture is made known. By the unique character of the ventral hump this 

 species is easily recognizable. 



Ophiopluteus retrospinus nova forma. 



The main distinguishing feature of this larval type is afforded by the 

 peculiar backward projecting processes from the lower part of the postero- 



Fig. 89. Skeleton of Ophinpluteus relroxpimis. species a. "%. Letters as in fig. 85. 



in. median process. 



lateral rods. - - The arms are fairly long; the posterolateral arms are about 

 three times, the anterolateral arms only a little more than body length, 

 their point being at a level with the point of the posterolateral arms. The 

 two other pairs of arms are somewhat shorter, their points not reaching 

 the same level as that of the other arms. There is a very slight widening 

 at the point of the arms. They are all gracefully curved, especially the 

 posterolateral ones, which gives a touch of elegance to the shape of the 

 larva. The frontal area is well developed. The preoral band is somewhat 

 upwards curved, more or less straight in the middle. The postoral band 

 makes a little downward curve at each side (more distinct in species b), 

 and from here the nervous system is seen to proceed. (In species b only 



21* 



