408 Mr. W. P. Sladen on the Structure of 



pair or two of pores are scarcely larger than the usual Ophiuroid 

 tentacular orifices. This circumstance, together with the fact 

 that the pores in the neighbourhood of the actinostome are 

 likewise somewhat more circumscribed in breadth than those 

 in the middle of the radius, imparts the petaloid form to the 

 outline of the poriferous area. 



It is also of importance to note that there are no tentacle- 

 pores or tentacle-scales present on the rudimentary arms. 

 Owing to the state of preservation of the specimen under 

 consideration, it is impossible to determine what was the length 

 originally attained by these aborted prolongations of the radial 

 system, as they have been unfortunately broken in every case ; 

 but from the rapidity with which each succeeding joint dimi- 

 nishes both in size and character from its predecessor, as well 

 as from the rudimentary nature of the parts, the probability 

 would seem very doubtful that these stunted, undeveloped 

 arms were ever of any great extension. As indicating the 

 really diminutive size of the small free rays, it may be men- 

 tioned that the first three segments together measure little 

 more than one tenth of the diameter of the disk. 



The innermost or basal portion of the pore-area is restricted 

 on either side by a narrow elongated plate, which joins up to 

 the alas of the first under arm-plate and is directed downwards 

 towards the side mouth-shield (mouth-plate) ; it is somewhat 

 sigmoid in shape, thickened at the outer (aboral) end and ter- 

 minating inwardly in a point, and forms the interradial margin 

 of the first tentacle-pore. This plate I am disposed to regard as 

 the homologue of the Ophiuroid genital scale, all hough in the 

 present case it is comparatively small and insignificant, being- 

 even shorter than the first under arm-plate. There appear to 

 be traces of a slit on the interradial side of this plate, between 

 it and the interradial plating, such as would correspond with 

 one of the rimce genitales ; but in the present condition of the 

 specimen it is impossible to speak with certainty on this 

 point. 



The communication between the oral cavity and the inter- 

 radial space is uninterrupted by any calcareous development, 

 so far as I am able to determine without injuring the speci- 

 men. By throwing a strongly concentrated ray of light 

 through the test, two small reddish stains, about 1'5 millim. 

 in diameter, are discernible in each interradial area, situated 

 on the sides adjoining the radii, and at about one third the 

 distance between the mouth and the margin : whether, how- 

 ever, these indicate the limits and position of the generative 

 organs cannot be stated decisively without further material ; 

 but from the position which they occupy, as well as from the 



