712 



ME. T. T. GROOM ON SOME 



(c) Of the third variety only a single specimen occurs, it was found 

 on the upper side of the test, near the middle of an interambulacral 

 area. It is much larger than the others, and measures 2 millim. in 

 length. Each valve has a stout basal portion perforated by a cir- 

 cular foramen, A prong given off from this widens out at the ex- 

 tremity into a spoon-shaped end, the margins of which are supplied 

 with a number of small triangular teeth (PL XXVIII. fig. c 6a). 

 This, and the first variety, may be referred to the tridactyle type. 



As this pedicellaria became detached from the rest of the specimen, 

 I was enabled to examine it from all points of view. Eig 3 b is a 

 side view, 3c a view from the basal end, and 3 d a view of the three 

 spathulate prolongations. The circular aperture is distinct only in 

 one of the valves. In general shape this pedicellaria resembles 

 those of Asthenosoina hystrix figured by Sir Wyville Thomson *. 



I have found pedicellariae of the varieties a and b on the specimen 

 in the Woodwardian Museum. 



Pedicellaria-stems (PI. XXYIII. figs. 4 a, b, c, d). — Numerous ex- 

 amples of these are scattered about the test. They vary greatly in size, 

 but all agree in having a slender shaft with enlarged extremities ; 

 the basal extremity ends abruptly, while the free end to which the 

 pedicellariae were attached is well rounded. I found none in actual 

 connexion with the pedicellariae ; but they are readily to be distin- 

 guished from the spines ; they are more slender, and the striations 

 on them are much finer. The rounded ends have no constriction 

 such as is found among the Echinothuridae and Diadematidae. The 

 stems more resemble those of the Echinidae f. 



Teeth. — I have lastly to mention, the teeth. These were figured 

 and carefully described by Wright, in his ' Monograph of the Oolitic 

 Echinodermata ' J. Mr. Keeping, when describing PeJanechinus, was 

 unable to find the specimen figured by Dr. Wright. In company 

 with Mr. Newton, I succeeded in finding it in the collection of the 

 British Museum. It is undoubtedly referable to our Pelanecliinus 

 corallinus. The epiphyses were probably not united to form an 

 arch above the teeth. 



Affinities and Systematic Position. — Of the distinctness of Pelan- 

 echinus as a genus there can be no question. While it resembles 

 the Echinothuridae in the flattened shape, in the flexibility and thin- 

 ness of the test, in the length, undulating boundaries, shape, and 

 general characters of the interambulacral plates, and above all in the 

 nature and arrangement of the peristomial plates, yet in the con- 

 trast between these and the adjacent coronal plates, and between 

 the ambulacral and interambulacral plates, in the notched peristome, 

 and in the types and variety of the pedicellariae, it is allied to the Gly- 

 phostomata of Pomel (Echinidae and Diadematidae). 



The flexible test with overlapping plates occurs also in Astropyga 



* Phil. Trans. Royal Soc. vol. 1G4, pi. lxiv. figs. 4, 5. 



t See figure of Strong ylocentrotus in Al. Agassiz's ' Revision of the Echini,' 

 pi. xxiv. fig. 20. 



% P. 163, pi. xii. fig. 1 a. 



